For Holland it'll do in the luggage, the police don't care ,only the in-sewer-ants will use it to squirm if the worst happens and then I bet they'll find better reasons if they are that way out.
If you were heading the wrong side of the pyrenees or far enough south/east where they don't pay their coppers, there are two options. Get a new plate with the EU stars and GB in the centre or display the GB. It only has to be visible from the rear which means you can put it on your luggage or your self.
How about laminating a sticker and making some sort of clear pocket that'll hold it in your back pack/cargo net/ relective belt?
BTW, Forget about the fire extinguisher and triangle too, nowhere civilised expects you to carry them. The bulb kit is important to French coppers as they traditionally got a cut from any garage they called out to replace a bulb and old habits stick long after the bribes have gone. The first aid kit is useful to yourself, so I carry one. Headlight deflectors depend on your lights and when you'll ride. If it's daylight/summer only and your running light is just that, I don't bother. Tape will do if someone does claim your dazzling.
Andy