I agree with much of what SultanOfSwing has suggested above.
You really need to be in a job to get through this.
But sleeping in a permanently stationary vehicle makes you very vulnerable, so you need to do that for as short a time as possible, so you hopefully don't get noticed.
When I was homeless, I sometimes slept in parked up trains, semi derelict buildings, lobbies of flats and squats, but that was a long time ago and I don't know if these are available now. There was also the mixed blessing of other homeless people; it was possible to form alliances and friendships, for safety in numbers, but many were untrustworthy and made it worse.
If you can save, maybe get a loan or an advance, and manage to buy a cheap old (mobile) van with a long MOT, you are already in a better position. You will be more comfortable sleeping and less exposed.
You can move around and park unobtrusively in different locations, plus drive to the leisure centre for your ablutions.
It doesn't have to be large, something the size of a Kangoo or Belingo works, depending on how tall you are. (You can sleep diagonally)
Always park so that you can drive straight out of wherever you are, without having to reverse, for a quick exit when you need one. It helps if you can get to the driver's seat from the back. I found that ordinary streets with shops and houses, but away from pubs, were safe areas rather than parking in leafy suburbs with Neighbourhood Watch, or isolated car parks.
As a lone woman stealth camping in a van, I found it safer to settle in for the night earlier than to go to my van late at night, much less noticeable. (I could have some led lights on, unseen, to read)
Waking earlier, especially when it was colder, meant I could move the van before most people started going to school or work.
Finding an early opening cafe to get warmed up with tea and unstiffened before most people started the day helped, too.
I would say that one of those travel wee containers is an essential item, so you don’t have to exit the vehicle in the middle of the night.
Good luck