The Mac bought in the US will have an American English keyboard, vs. the British English keyboard you'd get in the UK. Two countries divided by a common language and all that, but they're not too different (return key shape, £ instead of # over the '3' so you have to use option-3 to get £ instead of shift-3, little things like that). Everything else about the Mac itself will be the same regardless of where you buy it (except probably the price...and if you need to pay VAT when you declare it at Customs...).
The power adapter will come with a North American duckhead connector, but the adapter itself is universal voltage (100-240 V, 50/60 Hz so you need to change just the plug, there's a British duckhead in the Apple World Travel Adapter Kit - Apple).
American English

British English
