The problem with being airside of immigration in the UK insofar as HMRC is concerned is that there is nothing that covers that particular situation on the statute book. HMRC can interpret your 'being in' or 'visiting' England in basically any way they wish to. It would then, if it came down to it, be up to you to prove that your interpretation of HMRC's own rules was more correct than their own, in a court of law. This is sometimes a little tricky as Mr Gaines-Cooper found out, to his significant cost and, incidentally, to the cost of many others.
The jurisdiction of British law certainly extends airside of the immigration desk, just try telling the next hostess who smiles you off her flight that she's a somewhat less than attractive and talented tribute to the traditions of airborne female servitude!
