I thought they worked in airports (and terminals) which support them. I was under the impression that VS (so thus T3) and the likes of Vegas (actually that far back it was probably LGW or MAN I flew from) supported them. But indeed, I might have just been lucky a couple of times.

But T5 doesn't, but it might be ok if you're flying in from an airport which does, the departing airport being the important part.