Originally Posted by Freddorick

Any airport that has an e-gate will usually do. I have done this many times in HEL (to enjoy the somewhat better although still meagre offerings of the non-schengen lounge). ZRH, DUS and FRA also allow for transfers between Schengen and Non-Schengen area if memory serves right. Madrid at least used to work in the past.

The thing to note is that this mostly works when no human contact is involved. If you have to see an agent, then they might be unwilling to let you go from Schengen to Non-Schengen for the puposes of “lounge hopping”, but they can’t stop you on the way back as long as you hold an EU passport. And I suspect the same applies if you hold a valid visa and have a sufficiently long layover.

I don’t think LHR is a hack. You are a passenger, not a prisoner. I’m sure that the airport would like it otherwise though, after all they would rather that you spend your money in the terminal.