Years ago, I worked for a company that employed writers and sold essays and dissertations. Not as a writer, in the office. (Long story, I didn't know that's actually what the company did when I got the job!)
The trouble is there are sooooo many foreign spam and scam companies out there that rip off students by trying to look like the main uk companies. So I'd be very wary about spending hundreds - potentially thousands - on a dissertation and not receiving anything, or receiving a piss poor piece of crap back. If it's a spam or scam company, you're screwed. We saw that many people who'd been ripped off previously. (Especially because the scammers would make their sites seem like part of the company I worked for's uk group).
The other thing is, fast forward a few years and i know somebody who works as a lecturer. They can spot when it's not their own student's work a mile off. Turnitin etc not needed.
So, why take the risk of spending all that money for it to be a waste? Then, it will undermine the rest of the work you've already done for your degree if you get taken to a plagiarism forum and given a zero mark. It'll drag your average right down and you'll walk away with a crap mark overall. Or thrown off the course with no mark!
I wouldn't risk it. Best thing to do is email what you've said above (minus the bit about buying a dissertation) to your supervisor and ask for an extension. Listen to his/her feedback and do the rewrite.
It seems impossible now, but I promise it can be done. You're stressed and need a break. Your university will grant you a little break!