We are your fellow end users, neither Apple nor tax officials. That is far beyond what can be found in these mutual tech help forms.
However, warranty info is publicly available. Are you aware that the standard one-year warranty for iPhone is only good in the country in which it was purchased?
Legal - Hardware Warranties - Apple
As far as I know, the standard one-year warranty on Mac computers is still international BUT AppleCare+ for the same device is the same as iPhone, i.e., only good where the device was purchased.
So if you want to buy in the UK to save money, those saving may well be a moot point if you have to fly back to the UK for service or pay for non-warranty service in your own country.