OP,
I lived in my car for 9 months. It was rough. But, I managed to find loos, learned how to sink wash in those loos... early doors is best.
To live in a car takes some resilience. Have baby wipes (and dispose of them properly) nappy sacks, take out a National Trust membership and park up. The UK isn't set up for people living on the road, there is an inherent suspicion surrounding people that live on the road.
Having said that, I met some fabulous people, I ate well, yes it was cold at times. But, I had good outdoor wear, that gorgeous me through the cold nights. I learned how low to put my window, to avoid condensation, I learned how to manage in extreme circumstances, Greggs breakfasts was a saviour, I learned how to know which service stations would help me out and how truckers would park their cabs around me so I was safe.
I worked ground jobs, seasonable jobs and saved a shedload of cash. Having read some of the responses here, I'm almost laughing out loud. Only MN can be so horrified at such a thought.
I'm in my mid 60s now. This was 2 years ago after I lost everything from some poor business decisions. I miss that time in my life. But, I was at my most healthiest. I still had a bank account and managed to function with odd jobs and had an address that I paid for.
I'd recommend investing in a good all weather sleeping bag, I lived in a hatchback where I could put my seats down. I had a kettle, a collapsible loo, I pooed in a nappy bag, then disposed of it in dog bins. I used wipes to wash with, put my make up on in the visor and paid my mobile phone bill every month. I still have my hatchback that saw me through those 9 months. I worked in hotels, (desperate for staff) then moved on, after a pay packet. Saved nearly 3K.
I'd make sure you invest in good tyres, that your car is in tip top condition and don't run your engine too long as that drains your fuel.
If this is what you need to do to save some cash, then do it. Learn how to manage your safety, how to manage your needs and where to brush your teeth. Always, always have a bottle of water with you.
Good luck.