Are you in your first year? If so you may be within a 12 month probationary period. You need to dig out your contract (or request a copy from the school office) and see what it says in black and white.
Then have a meeting with your union during work hours and discuss what your options are. Don't get fobbed off.
In no way are they allowed to say that Friday is your last day. That's under a week's notice. So that's a nonsense. This is work place bullying, you'll have a set notice period that applies to both you and the school so find out exactly what that is. They can't break that as they'd be in contractual breach and you could sue.
Failing all the above, and with your mortgage to consider, I'd accept the formal capability and have outlined clearly what your targets are. Then work diligently toward achieving them. If the school claim you've failed then challenge them and ask for an independent point of view. Under no circumstance attend a meeting about your capability without your union rep present. Not even a quick 'can I grab you for 5 mins?' chat as you're walking past the office door. Request all meetings are set up via your rep and with at least 24 hours notice to you.
At the very least this will delay the whole process enough to mean you're paid through the summer. Then you can resign and look for a new job. Supply as you're looking for full time employment.
It's rubbish that it's come to this, but take time to reflect on your teaching. Is capability fair? Have you had a series of poor lessons that have been observed or is it a case of them constructing something to get rid of you as their budgets contract? If this hasn't been signposted (and I'm talking over a prolong period such as a term, not a fortnight) then you may have a case for constructive dismissal and should get a lawyer from your union. It sounds like you may need to stamp your feet a little, rather than being so accepting of it.