Indeed, you can still preorder from nokia.co.uk. if what expansys said is true, that Nokia closed a contract with Vodafone UK for N900 and all their first shipments will go there it doesn't matter that you can still order from nokia.co.uk.
The change that MPD made is strange IMHO.

I can understand Nokia on this decision, such a contract values a million times more than the couple of thousandths of pre-orders and unsatisfied users. Delaying the shipments by a week or two is not such a big issue for them, considering shares value and what a plus can a contract like that bring.