Can't think of many alternatives, if it's accessible then you might want to ask about getting a plate welded on, unlikely to get an MOT if they see the hole.
You might get £200-250 as spares or repair which isn't much of a deposit for a replacement, you've already committed money to the car, so IF this is the only issue then it's probably better to bite the bullet and sort it.
(Assuming this is rear subframe?)