Brand new T28BBs can be bought from Apex Performance for ?575. Your only other hope is to find a 2nd hand one up for sale on SXOC. That's fairly rare and they go in a flash.
They are as rare as rocking horse crap to find 2nd hand in the UK. You could try www.euroautosalvage.co.uk and see if they are willing to take one off. Don't hold you breath though...