You are dealing with three very different legal systems of which I can only comment on the UK one with anything like proper authority.

UK national law does not specify what is or is not a helmet, EU directives do but these apply to sales more than use. This loophole was included so that people who had helmets when the law came in did not have to replace them. As a result the police will typically not stop you so long as you wear something that looks like a motorcycle helmet because there can be hassle if you claim you bought said helmet when it was 100% legal. However, wear some crappy plastic fake Nazi helmet or other Christmas cracker novelty and add a bit of spirited riding and they will stop you. They'll then start looking for something. An ECE or BS marked helmet puts a tick in box and they'll move on to the next item, insurance, driving license status etc. If they deem the helmet unsuitable they'll take it as evidence and you'll be getting a bus to the nearest bike shop or waiting for a specialist to determine if it is a helmet or not. Some coppers will bypass this and simply issue the ticket/stop you riding, it depends on how their chief constable views failed convictions on the ones that get a lawyer.

There are exceptions to this. Some local councils such as Westminster passed their own laws to make the police's task easier. If there is a byelaw stateing your helmet or visor needs an ECE or BS mark, you'll be getting a ticket in addition to the one for that bus.

If the helmet is marked "Off road use only" the ticket will follow double quick. A lot of "custom" hats brought into the UK have these added so the seller can't be sued by the dead blokes relatives.

In France and Italy I don't know the law. You see riders wearing all sorts particularly on small bikes. The police in Italy especially tend to take the view that young Marcello is only going a few miles so that's OK, but tourists should obey the letter of the law for their own safety. On the autoroutes the Gendarmes are quite likely to start a fishing trip if they think you look wrong.

IMHO you'll get away with it, but on the instance you don't you'll be in for a fair bit of hassle. Why not check out scooter shops, they'll sell you something as close to what you like as they can but with stickers that'll keep plod happy?

Andy