Anything with a battery in it is covered by their global warranty.
You’ll need a UK plug adaptor for the power supply. For most of their power supplies, one of these can just slide on instead of the US one.
Keyboard will be US, but there’s not much different (Apple have the @ and ” round the wrong way on the UK one), it’s really just different shaped enter key and no £ or € symbols.
The value exceeds the £390 allowance for personal import, so you could get stung for import duty and VAT if stopped at customs. Chances of that are very slim though.
Worth calling Dixons Travel (the ones in the airport) and seeing what price they can do. Compare against US price, including local sales tax; Apple prices are quoted without, and in NYC it’s 8.875%. IME there can be very little difference between the two, and from Dixons you get a UK model.