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Can Lord Woodesey have an antlered helm? Yep, you bet, absolutely. As Kilisi pointed out, lots of impractical/decorative armor existed back in the day, just because the wearers liked how it looked. Does Lord Woodesey really need it to be battle-worthy? Even if he does, allow me to point out that in a fantasy/RPG setting if you say "Lord Woodesey appeared, wearing a badass antlered helm", no one is going to say, "but are the antlers attached loosely enough to prevent brain damage?"

However, since you've asked, and it got my imagination firing, I came up with a few (uneducated) attempts at making "easy-break less-head-trauma" helmets. Many might be impractical if tested, but it was fun to dream. And even if none of my ideas would actually work, I have no doubt that it is possible to do what you are asking. A good engineer could probably solve the puzzle fairly quickly. Would it ever be quite as safe or efficient as a normal, practical helmet? Unlikely. You have freakin' horns sticking up off your head. So, with that out of the way, here are my best horn-hat catastrophes:

...or we could be boring and just glue it...

Yes, absolutely

Lord Woodesey and I seem to have had the same idea, great minds think alike, so they say. There are several ways to have a deer antler helm without it being detrimental.

The easiest, and by far the simplest, idea is to fix the antlers to the helm and then cut through them near the base, leaving you with two nubs attached to the side of the helmet. Then, you glue the antlers back on to those nubs (glue did exist in Medieval times but you may also be able to use wax if you prefer).

This may seem redundant but, by cutting and gluing the antler, you have created a structural weakness. This means that, when force is applied from someone pulling it or a weapon strike, the antler breaks off where we glued it to the base. This prevents someone being able to use it against you but still allows for you to wear your antlers into battle. The plus side of this is you may also be able to re-glue the antler back on after the battle if you pick it up and its not too damaged. A similar thing can also be done with horns, using the exact same process as described above.

Metal though is a little bit different. Due to its weight, it would be a lot harder to use glue to achieve the same effect with steel or bronze, possible with modern glues but less so with Medieval ones. Instead, what we can do is make these pieces out of a lighter material such as aluminium or tin and weld the pieces together. This means they do not add too much extra weight, can be broken off easily due to the structural weakness and can possibly be repaired or replaced after the fight.

Alternatively

Rather than risk Lord Woodesey damaging the antlers (or whatever else he chooses to decorate his helmet with), we can instead have detachable horns. Essentially we can have something similar to arrow or spear heads. A metal fixture goes onto the side of the helmet and the antler or horn is pushed into that fixture (it has already been filed down to make sure it is the right size and a snug fit).

This way your antlers can easily be pulled out of their fixtures before battle, so they are not a hinderance, and put back in when not in combat. This also prevents them from being damaged or lost in a fight. You could even do a similar thing with metal.

You could do it easily, it still won't be as effective as an unadorned helmet, and I don't see the need to wear it in an actual battle.

There was lots of ornamental armour around back in the day, and the Japanese Samurai armour had all sorts of projections on helmets. So it's not a showstopper. The projections were thin soft metals that would crumple or get chopped off if hit. So something as simple as attaching the antlers to a thin soft metal holder would do the job.

Kilisi was right to bring up the Samurai helmet, but missed the most important point: the adornments on Samurai helmets were not just ornamental. They were designed to work like a combination of sloped and ablative armor making it almost impossible for a weapon to strike a samurai's head without being mostly deflected or absorbed.

To further explain the 2 mechanics at play:

Sloped Armor: When you strike armor against a flat surface or inside of a crease, you have the best chance of penetrating it because all of your force goes into the armor, and you penetrate it in the shortest path through it. When you strike armor at an angle, a portion of your force ricochettes away from the target, plus the armor is "thicker" than going straight through it. So, while antlers might not be ideal here because there are so many creases to catch, there are various horn shapes such as the those of ampala, rams, or antelopes which will almost always create some degree of a ricochet when hit.

Ablative Armor: Armor designed to bend and shatter when hit will distribute an impact across the shatter zone instead of letting it focus into one spot. So, if for example, had a ram's horn on the side of your helmet, and a battle axe hit it, part of the axe's energy would go into the impact, and some would go through the impact to hit your actual helmet. By decelerating the impact over several inches and distributing it across the shattering horn, the strike will be far less harmful than if the axe were to suddenly impact the side of your helmet.

So, instead of worrying about how to make antlers fall off as a whole, I'd try to make them ablate impact as best as possible. To do this, I would drill holes at the creases so, that it shatters at a point before driving too much force down into his neck, but giving enough resistance to slow the weapon that hit it.

It depends.

The antlers can be attached to the helmet, so that it takes a certain amount of force to break them off. The exact amount of force can be chosen for your needs.

The amount of force to break them needs to be high enough that they don't break off by accident. Google says deer antlers might be 1.3kg (3 pounds), and might be 60cm (2 feet) (not the same ones antlers, just for an idea of scale).

If his lordship were to turn his head quickly (especially in the middle of battle), that would apply a fair amount of force to the base of the antlers. Obviously, it would need to withstand that, without breaking. If an attacker were to apply exactly this amount of force to the antlers, the helmet would transfer to his lordship.

The antlers themselves can withstand the deer (or moose, or whatever), so that's not a problem.

If they are not used for actual headbutting, and depending on the weapons technology, this may not matter. With guns, muskets, spears, bows and arrows, swords, etc, the chances of an attacker getting close enough to grab the antlers would be very unlikely. Especially if Lord Woodsey is on horseback.

In wrestling, of course, it's a different matter.

I would tend to modified antlers - if you can embed something sharp into the surface (think glass shards, razor wire), then anyone grabbing the antlers with bare hands will come off second best.

I would say, it depends on the actual conditions of the battlefield. Do people fight on foot or in horseback in this setting. What is the average length of a typical sword.
If we assume bronze to really iron age, with big shields and short stabbing swords, an antlered helmet is viable, especially as it would be as much ritual as functional.
If the conditions are similar to the high middle ages, and Lord Woodesy is a knight on a horseback with a lance, he can have antlered helmet as a fancy - he most likely would not be hit from above, and against the other lance the form of the visor of his helmet is more important. Jousting helmets if high middle ages often had different decorations, including horns.
In most other cases antlers would make sense only if they are easy to snap of the helmet. I do not think there was any glue strong enough to hold horn to metal before advanced chemistry. I would try using brass or wood pins to hold them instead.

Yes. The answer is in the fins. FCS II surf fins. Check it out. https://www.surffcs.com/pages/fin-systems

Firm, works side to side. Upon impact, they release. Sounds exactly what you want.

Lord Woodesy wears his helm as testament to his superiority. He can fight with a ludicrous adornment and inspire his followers. He has developed a fighting technique to deal with this.