Very interesting, thanks for the replies everyone. A bit scary how quickly anything that sounds nice to the ears is taken as fact.
politicfool, thanks for the link to exchange of notes, I hadn't seen that. The treaty always seems to say employer/personal plans are "qualified" as long as they count "for the purposes of Chapter ...... of the Income and Corporation Taxes Act of 1988", which as far as I read technically only applies to pension schemes approved by some mysterious board, which seems a bit of a confusing joke given how many plans there are. People seems to invoke this (and not) for their plan fairly often though, so a wild, wild west of interpretations.
Sigh..