Career advice: How can I move on from my probable PhD flop?

So thing aren't going well for me with my PhD and I sort of think I'm not going to get there (hard deadline is September and frankly I think I'm fairly close to having a breakdown). I'm in the UK and the hard deadline is university policy.

How soon should I start looking for work outside academia? How should I go about explaining my (likely) failure?

I'm not sure I know what to do from here.

I don't feel confident about bringing this up with my supervisor. If I did, he would brush it off (he doesn't really get me, I don't think) with something along the lines of "What else are you going to do? Just get on with it.". He has expressed concern about my progress before now. Also, part of the motivation for asking this question is feeling well-researched on this topic prior to having a conversation, which I'm hoping will show my supervisor that I'm serious about it.

If this information is useful I'm based in the UK, I have a pass at Master's level already and a 2:1 BSc in physics and I am in an allied field at the moment.