In future, keep the headlight deflectors in your glovebox. The bu**ers are never wearing them when they come over here. And seldom do you see an "F" sticker on a French registered car, I have only seen them on UK cars (presumably French ex-pats)
Cars dip downwards usually now I believe. They do give more light out to the left to light the roadside, but if you get stopped by a gendarme you could pretend you misunderstood some advice about the vertical dipping thing and don your deflectors then.
I would think some isopropyl alcohol should get the gunk off
While I am ranting, why do you see "Ecosse" stickers on Scottish cars? GB is the legal requirement, but if you are proud of your gaelicity why not have the word for "Scotland" but in gaelic? Proud Welshmen don their Cymru stickers, not "Pays de Galle". I can't wait to see the first Cornish separatists with "Kernow" on their cars!
Just reading up on Ireland, apparently the official language of Eire is Gaelic, yet most people can't speak it! Although I suppose the same could be said of English in many parts of England (least of all Albert Square)