It is certainly a good idea for US vistors to bring a guitar with them on a UK visit, to unload and help defray the cost of the trip. I imagine Mudcat might be a good place to line yourself a buyer before you set off. Similarly a UK musician heading for the states might well consider going guitarless, and picking one up immediately on arrival, and bringing it back afterwards.There may be some small questions of customs duty being payable, but as the officials are probably very busy there is no point in burdening them with extra work.