Calling all armchair generals! Boudica's Last Stand.

[Renatus]

I'd say their standard 30km/18 mpd scale is more likely - 10 days Deva to Londinium. Mona strait would add another 3 days to that - all very much in line with the estimates we've been using so far. Up the pace to 20 miles a day and they could do it in 11.5 days.

I take it that this is the route down Watling Street.  I calculate it slightly differently.  I make the distance from the Menai Strait to London as being c. 270 miles which, at 20 miles per day, works out at 13.5 days.  However, I don't favour that route.  Tacitus tells us that Suetonius travelled through the midst of the enemy.  Much of the route down Watling Street (in its latter stages, anyway) would have been through the territory of the Catuvellauni, whose tribal capital Verulamium had recently been made a municipium.  The tribe may, therefore, be regarded as having been principally pro-Roman.

As I have said before, I reckon that Suetonius' plan, having heard of the threat to Colchester, was to take the 14th Legion, rendezvous with Cerialis and the 9th and together go to the relief of the colony.  This would have taken him along the Via Devana, with a likely rendezvous point at Godmanchester.  Cerialis jumped the gun, possibly because he realised that Suetonius could not get to him in time, but even so was too late.  My theory is that, when he heard of the fall of Colchester and the defeat of Cerialis, Suetonius proceeded to Godmanchester (if he was not already there) and then down Ermine Street to London.  This would take him through the fringes of the territories of the Iceni and the Trinovantes and thus, with a little rhetorical exaggeration, through the midst of the enemy.  By this route, the distance from the Menai Strait to London is c. 298 miles which, at 20 miles per day, works out at 14.9, say 15, days.

As to how long it would have taken for Suetonius to get news of the threat to Colchester, I calculate that, with sufficient remounts and relief riders, a message could have been got to him within 24 hours.