Great idea and that works for you 
But personally I’m not keen on using those stickers/magnets and that’s why I’m trying to establish an approved number plate design.
OK I’m a bit OCD 
, but I know I’m not the only one
Yep I am a bit OCD too with cars.
Under normal use for Europe I wouldn’t use a GB sticker. (AS you don’t have to).
BUT as we may be popping off to Spain in due course a great idea as you have to have one regardless.
Glad the EU has standardised it all… 
One thing that will go in September are plates with raised letters. Plates must have a smooth flat surface. No more Gel plates.
I don’t think you’d be able to use a magnetic “GB” on an ND, there doesn’t seem to be any ferrous metal in the necessary area, correct me if I’m wrong.
Nope tried everywhere in 2018 when we went on a raid. Stuck a mini numberplate sticker (as mentioned above) instead. Have a couple in the foreign travel bag in the boot, warning triangle, yellow jackets, first aid kit, breatherliser etc etc. and will update to a GB/UK flag version to replace the GB/EUR one. 2 for £2.99 post paid eBay.