The workshare was/is based on the number of aircraft purchased. The UK government would have to in theory also compensate company such as EADS, Ellectronica etc. for taking work from their share. It probably would cost the same as having them if not more to cancel T3
Thats why the Germans are still in, some years ago the German government decided to see if they could pull out of the contract but it was going to cost as much to come out as it would to stay in, I believe they are happy now that they did stay with the aircraft. Allthough I think the Austrian Typhoons are actually from their 'quota'.