Just a few questions that spring to mind:
What is your short term tenancy and what is the reason the landlord has given you for not being able to stay there and extend? Do you have a contract with them?
You did not get a CCJ for your car being in a service station car park for an hour longer than it should have been. The private car parking enforcement company did not just send one letter that you say your friends didn't open and then you ended up with a CCJ. That's not how it works.
Why is it your long term plan to try and get the CCJ removed? That needs to be a priority, how long has this situation been going on for and what communication has occurred so far? You can't just not pay it.
If you were overseas, who left your car at the service station?
Do you have a job at the moment? If so, what were you going to do about your living situation anyway, if you are staying in the same area, the job move has not precipitated the homelessness, it was coming anyway.
And either way, why wouldn't you have been looking for a job somewhere, (anywhere) else where you could also get accommodation?
What will happen to your post in a couple of weeks? Do you expect the new tenants to open it and or let you you know?
Where does your insurance think the car is registered to. Once you are homeless your car insurance will be invalid. So you won't be insured to have it on the road. Where will it go then?
When is your MOT due?
What happens if you need the toilet in the night?
I really think you need to sit down and make a new plan.