I think the benefits of wearing a helmet in any form of aviation outweigh the risks by far. But there is the issue of personal image, i think younger people tend to want to look more "flash" and while the older middleaged types try to tone down their appearances, except when middleage crisis is involved. The average PPL (at least the regular ones) tends to be in the slightly older range and there is a general view that any extra aviaton kit is a sign of showing off. Combine that with a bit of the Great British Sense of Modesty and I can see why the orginal poster is facing the problems he's facing.
People will go to all kinds of lengths to put you down and you'll get all kinds of ludicrous statistics. FACT: many more pilots have died as a result of not wearing a helmet than have died as a result of wearing one, never heard of the latter.
Since the field of aviation you intend to work in requires you to wear a helmet, i think you are perfectly right in your decision to wear one now and your insructor's attitude is quite appalling.
Good luck with your flying
Capt. M