Usually once an hour a BA (or sometimes BAA) employee will escort a group of self-landers out, via Border Control to landside. Made it a bit difficult for a chap who was on the same LHR-MAN flight as me and self-landed without a passport, but in the end there were no issues.

All in all, a very common situation in T5.

P.S - If it was such an emergency e.g. your wife was giving birth etc in T5 airside, I'm sure you wouldn't be hanging around long!!