Company Car Vs Allowance

Any

That is right. Employer cannot expect you to use your own car for company business and pay you just fuel. Especially as if that the case and you use your car regularly you need also better insurance to cover you for business use. Normal insurance is not good enough.

As I say, you can get tax relief on the difference but that will very unlikely cover all of the costs.

Some companies do have a "pool" car for these situations.