Also Spain (and Malta?) require a UK sticker regardless of number plate markings. As per the post above, note the Euro star/GB number plate is not "illegal", you don't need to cover anything if you don't want, it just doesn't satisfy the requirements any more. Just add a UK oval to the back of the car.
Like many things regarding road laws, you are only likely to have an issue if you are involved in some other incident/accident/offence/attitude failure. I've been stopped at routine checks many times and as long as you have the obvious stuff no issues, paperwork, V5C, insurance, MOT (yes, even if the rules are different for that country - bikes have no MOT/CT in France but a UK registered bike must have one if over 3yrs old), hi-vis is best kept visible through the windows then they won't even ask. You should have spare glasses if you wear them, stuff like that is mainly just sensible, and wear the hi-vis if you need to get out of the car on a highway!!
Bulbs, yes I do like Pogglefish suggests and have "some bulbs" which look like they're useful. Never been questioned.
With ANY European police officer, be very polite. You will NEVER win an argument. They can do stuff like impound your vehicle until you pay a fine, in cash often. They can make your life difficult if they choose to. Smile.
Some countries are a lot stricter on the country marker (UK sticker) than others. Austrian police WILL fine you for not displaying one, French might be more "laissez faire" (they invented the phrase after all) but personally I never tempt fate for such trivial items. I have CritAir stickers even though I generally keep well away from bigger cities, but think how good it looks to a gendarme if you've made the effort (easy to apply online, just need to send an image of your V5C and a few Euros).
https://www.certificat-air.gouv.fr/en/