Originally Posted by David_Doyle

A B2B is pretty safe as you will have arrived on the departing aircraft (of course).

For transiting it's quite straightforward and I think there's been some comments on here. When you get to passport control you tell them that you are in transit and they will escort you through a check back into departures. There may be some questioning, especially as you're departing back to your origin point which did raise a few eyebrows but all was fine.