Calling all armchair generals! Boudica's Last Stand.

[Renatus]

Why didn't the residents of London and/or St Albans simply leave without military support if the area was pro Roman and safe?

To add to Nathan's post, all we are told in the sources is that some could not or would not leave London, while some put themselves under military protection.  It is likely that others had left but, I suspect, only those who had somewhere to go.  Many may simply not have travelled far outside London and did not know what was out there.  News would only have passed by word-of-mouth, so all that many would have known was that there was a threat approaching but not how widespread it might have been.  The horror stories may have been circulating, so again many would not have wanted to venture into the unknown.  These are the people most likely to have put themselves under Suetonius' protection.

So far as Verulamium is concerned, we are told nothing of what the inhabitants may have done.  Again, some may have sought to put themselves under Suetonius' protection, if he would have them.  However, their situation was somewhat different from that of those from London.  They were part of a large tribe and may have made contacts through tribal assemblies.  Moreover, as the tribal area extended far beyond the confines of the town, they may have felt confident that, wherever they went in that area, they could expect a sympathetic reception.