Fuel expenses/mileage allowance

Blingy2019

I have been looking into going self employed. I can potentially do some work as substitute driver.
I would need to use my own car to travel to do the work, do I claim mileage allowance on the cost of getting there and back.
The job would be courier driving I have 2 options
1. Buy my own van or lease van etc (cannot afford to do this and currently have no credit rating as been out of work for a while looking after my children)
2. Pay to borrow a van from the guy who wants me to do the work. (much cheaper option as van is already insured for the correct usage and anyone over specific age can use)
Can I claim the money back that I pay to use the van also I would need to put fuel in myself so do I invoice the guy for the fuel I put in then claim the fuel as an expense?
Can you claim for business usage on car aswell as cost of borrowing the van and fuel?
I have done tonnes of research but not sure I can pay someone to borrow there van or not. Its a very grey area.
Do not want to get things wrong and end up been way out of pocket.
They would of course declare the payments they receive off me for use of van.

Blingy2019

I have already looked into this but I don't have to do it if I don't want to and I can send someone else in to complete the job aswell. Few others at same place who have said if I go into self employment they would like me to cover there holidays etc aswell so I can pick and choose who I work for and what days etc. It pretty much lands in grey area not fussed either way just wanted to check about mileage fuel costs etc. They all work for a company on a self employed basis sometimes they just want a night cover sometimes a week sometimes a month sometimes longer. Hoping at some point to get my own van etc and have another stream of business doing other work too but for now want to keep it simple and see how things go