20 seats in question, as claimed involved, does not invalidate the entire parliament. if you want to go down this route, all other legislation is affected, including the Finance bill, so no income taxes to collect.
seems an odd situation for a constituency MP to be responsible for expenses they may not have had any control over, coming from central party planning. for one thing, its ripe for manipulation as long as your "seen" to not be spending in the constituency, example of Ramsgate, if they'd stayed...