The best way to describe where to fit them comes with the deflectors themselves, there's a big piece of paper with the diagram for each different light type - the Coupé is included.
I just got mine on the ferry, about £5 iirc. Also available in places like Halfords.
Not sure whether lowering the lights to 1 would solve the problem or not, sorry

I know in theory the problem is that UK car light specifications mean the lights aim slightly to the left to avoid glare in this country, meaning they are shining directly at cars on the other carriageway when abroad. Whether lowering the beam is enough to prevent this, I'm not sure.