When an N reg plane visits the UK then of course it keeps its altimeter; how would airlines manage if you had to change?
However, if you take an N reg PA28 and bring it here to put it on the G reg I think it then has to have an altimeter in Hpa fitted. I say "I think" because I can't quote the regulation and I may be wrong; perhaps someone else can confirm.
I agree, it's a real pain changing units all the time but it's just the way it is! I frequently fly to the US where I get my fuel in USG, record the uplift in Litres on my tech log and then look at my fuel gauge which shows Lbs!
Always the possibility of an error when making conversions so robust cross checking procedures are very important. You set your altimeter then checked against the field elevation and spotted the mistake; absolutely the right way to operate. Uploading 1,000Lt of fuel when you really wanted 1,000USG could really spoil your day!
Happy landings!
3 Point