always get the gb sticker or appropriate plates, because that's easily visible to the coppers.
Don't bother with the light deflectors though, that's just a scam. Always makes me smile seeing people buy them on the ferry.
The only thing that i've ever been done for in about 20 years of going to europe was not having snow chains during the winter months, that was in italy but the guy just waved his finger at me and told me not to do it again. I think in theory i stood to face a fine of a few hundred euros on that one. They were stopping everyone on the border though to check they had them, i seemed to be the only person who hadn't got the memo.