Calling all armchair generals! Boudica's Last Stand.

[Renatus]

Later we are given a figure of I think 70,000 dead, from the 3 towns which seems high but if Verulamium is not included then this figure looks fictitious at best.

Tacitus says that the 70,000 dead were citizens and allies in the places mentioned, i.e., Colchester, London and Verulamium, so there is no reason to conclude that Verulamium may have been excluded.  Although he confines these casualties to the three towns, I would not be surprised if the figure includes anyone else who got in the way of the rebels.

Tacitus then tells us SP met up with his reinforcements. This is written AFTER the destruction of Verulamium and Londinium, so now he has all of his troops and chooses to attack without further delay.

Tacitus does not say that Suetonius met up with his reinforcements.  He says that he has almost 10,000 men, made up of the 14th Legion, the veterans of the 20th and the nearest auxiliaries.  This, no doubt, represents some reinforcement of the troops that he took to London but is by no means 'all his troops'.  He is missing the 2nd Legion, the remainder of the 20th and what is left of the 9th, not to mention however many other auxiliaries that might have been available.  All these (with the possible exception of the 9th) were, on my analysis, on their way to him but could not arrive in time.  As I have said before, Tacitus makes a virtue of necessity in saying that Suetonius decided to put an end to delay and give battle.  He was forced into it by the numbers and proximity of the enemy and shortness of rations.  This is from Dio but is nevertheless highly plausible.  I don't know why you emphasise 'AFTER the destruction of Verulamium and Londinium'.  I don't think anyone is saying any different.