I don't think Derren would risk using stooges. If one person came forward with a contract or other definite proof that they were paid to "play along" then his reputation would be ruined, and the general public would assume that everything he does is stooged. Also, broadcasting regulations are quite strict in the UK, and Channel 4 could be fined for broadcasting the disclaimer "At no point are actors or stooges used in this show", if it wasn't true.
It's quite likely you'll see a couple of the same faces turn up every now and then in British magic shows, especially those made by Objective, because when they're looking for volunteers, they'll advertise in the same places, and if someone's interested in magic, they'll probably volunteer for several things. For example, the undertaker who Derren performs Out of This World with the "dead" and "alive" pictures (hopefully you know what I mean), also features on one of Barry and Stuart's shows. This doesn't mean he's a stooge, though, he's just an undertaker interested in magic.