Calling all armchair generals! Boudica's Last Stand.

[Nathan Ross]

given the Thames is almost impossible to cross... I could find no credible evidence of a ford across the Thames there...

You will see from the source I linked that the Thames is (or was) far from 'impossible to cross'. The ford at Cliffe, or another nearby, was known to the Britons (according to Dio) and was used by Caesar, the one at Lambeth might have been dificult to cross but was certainly not impassable, and in 1016 Canute was apparently twice driven back across the Thames at Brentford, it seems (Anglo Saxon Chronicle).

Whether you choose to believe these sources or not, you must surely concede that the Thames could not have been considered an impassable barrier, and even if Paulinus was inclined to surrender the entire province north of the river to the enemy and retreat to the coast he could not have done so with any safety.

Were he looking for a defensible position he would have headed for the nearest high ground, rather than split his army up trying to defend river crossings.

Which is almost certainly what he did...

I though St.Albans was North of London

I believe you wrote of *the* north, not just a northerly direction.

The attack on St Albans occurs chronologically after Paulinus's retreat from London, and so can neither have caused it nor influenced his strategic thinking at that point. 

I'm not sure where we got mixed up.

Apparently you wrote Belgae when you meant Brigantes.

The Brigantes, however, were not in revolt against Rome at this point. The tribes of north Wales had just been crushed by a Roman campaign and were in no condition to rise in revolt imminently.

The attitude of the Belgae to the Romans at this time is unknown. The Regii (probably) of the Silchester area were perhaps allies. The Catavellauni of St Albans were certainly pro-Roman, as the town had been made a Roman municipium. If Paulinus was looking for friends in the area, he would have headed for St Albans.

By the time he got to London, he didn't even know if his own army had also been attacked and wiped out. He was alone...

He had his troops with him. Tacitus tells us he marched to London 'with wonderful resolution', thinking he might 'choose it as a seat of war'.

his mad dash down to London

There was no 'mad dash' either to or from London. Surely we have at least established that by now!