BTFB
We have added GB plates on our bikes. We added them after being hooted at in Bergen, Norway. As the Norwegian guy who helped us at the time said there is nothing on your bikes to say you are British and with Ortlieb panniers guess where they think you come from! There are are still a lot of Norwegians with long memories, if you get what we mean
How I got around it was to buy a small stick on GB sticker from
Halfords and glued it to a bit of aluminum plate which I then cut around leaving a small tab at the bottom. I drilled a 3mm hole in this tab and pop riveted it to the mudguard and then bent it upright.