Calling all armchair generals! Boudica's Last Stand.

[Renatus]

Presumably if the sowing is in the spring they start their revolt in April or May, fight in June, July and August, and aim to be back for September, for the harvest?

I doubt that.  If the plan was to drive the Romans from the province, as we are told it was, that was likely to be a lengthy process.  If they withdrew to their homelands for harvesting and autumn sowing, they would be passing the initiative to the Romans, Suetonius would gather his forces and launch a retaliatory attack, and the rebellion would be at an end.  They had to keep the pressure on and that would allow for no relaxation.  Accordingly, they had to remain in the field until the objective was achieved and the longer they did so, the more tribes were likely to join them and the easier the task would become.

So since they're not actually starting the revolt in March (far too early for the Romans to be on Anglesey)... what are they doing instead? If they are actively 'preparing for war', why do the Romans not notice?

They were doing what they would have had to do whenever the revolt broke out: planning generally, preparing weaponry and organising their 'secret conspiracy'.  Why did the Romans not notice?  Why indeed?  Whenever the rebellion broke out, they did not notice.  So far as failure to sow might have been a warning sign, by modern reckoning spring falls between mid-March and mid-June.  Do we know when in mid-1st century Britain seeds were actually sown?  Perhaps, they made a show of preparing the fields without actually sowing anything.  In any event, if the Romans were to have thought that the Britons seemed strangely inactive, would they necessarily have seen that as an indication of an incipient bloody revolution?

Anyway, you may be right and now is the time to move on.