A laptop bought from the US would have a different keyboard (letters in different places) than one from the UK.
The customs people are aware of this sort of thing so can easily spot it.
It is NOT a good idea to buy a laptop in the US for use in the UK for all sorts of reasons but different keyboard is one, lack of support is another (if it goes wrong the day after you get back here the UK based company will not support it), modem differences, software differences and and on.