Should I buy an iPhone 6 Plus in the US and take it back to the UK?

My friend is a pilot and goes to America a lot, I was wondering about asking him to buy me an iPhone 6 Plus over I the US and bring it back for me as it is cheaper over there (disregarding tax). Will the iPhone work in the UK? Will she have to pay tax when taking it over? I already have a sim that I use in my iPhone 4s, will I need a new one? I'm assuming that the size for a sim has been reduced aswell.

You can, but it will be at the unlocked full freight price of minimum $660 USD in order for it to work in the UK. Yes. You'll need a nanosim and not the microsim the 4S uses. Just grab one from Virgin and you should be able to activate on your own at home. Possible import tariff when arriving in UK but I'm not certain. Best luck.