Question is - do we really want GOV.UK to have a stupid amount of servers standing by for this sort of event? i.e. people waiting until the last minute and then whinging about performance?
Ah... we could use the Cloud. And then another group of people would complain that Amazon/Google/Rackspace/MS had all their data in a location where the NSA could get at it.
Far better to offer a small sliding scale financial incentive to sign up early so that people would naturally spread the load.