Sorry, I should perhaps have been clearer. Yes, I went landside and came back in again. There was no obvious person on or near the transfer channel. My previous reference to Finnair was because the previous week at the commencement of my trip (which was not a B2B) I routed from Budapest back home via Helsinki and the security aspect if entitled to fast track (as one is on AY) was a breeze. Obvious being intra-Schengen there was no immigration involved. It was only on the final leg of the trip, where I did a B2B to return to LHR, that the absence of fast track became relevant, and then immigration came into play too

Anyway, I made it, and so it is doable, but I wouldn’t recommend it, just as in the B2B thread I advised against undertaking one at CTA, which is an airport I know better than BUD, but which I equally wouldn’t like to risk.