I ditched with a helmet (night, November, sub-Arctic).
The helmet was not hindrance and in fact was a boon in helping to reduce heat loss from the head, better than any touque out there.
Outside Europe, what I have seen, the wearing of a helmet is at the discretion of the pilot (offshore oil flying). Some wear them, some don't. I don't in the summer due to the heat. I didn't in the tropics due to the heat. I expect I will again in the colder weather. I know I should. My copilot is wearing one today.
Never ceases to amaze me what can become an issue in Europe.