Two separate things are going on here, so worth splitting them out.
If you own the campervan personally and use it for business trips, you claim mileage at HMRC's approved rates. For a car that is 45p a mile for the first 10,000 business miles and 25p after that, not 55p, but do check the current rate as it can change. Those rates are meant to cover running costs, so you cannot also claim fuel or repairs on top.
If the company buys it, the mileage rates no longer apply. It becomes a company asset, and HMRC usually treats a campervan as a car for tax. That matters because you only get writing down allowances, not the Annual Investment Allowance, so the tax relief comes through slowly. There is also likely a benefit in kind on whoever has it available, even before they drive a private mile, because the test is availability, not actual use.
You can avoid the benefit in kind if it genuinely qualifies as a pool vehicle: available to several employees, used only for business, not normally kept at anyone's home, and any private use is incidental. Those conditions are strict and HMRC does look closely, so keep clear records of who used it and why.
VAT is a further wrinkle. Recovery on the purchase is usually blocked if there is any private availability, so factor that in before buying through the company.
For most people, with one van and occasional business use, owning it personally and claiming mileage is simpler and cleaner. The company route can work, but it is more admin and the reliefs are weaker, so it is worth running the numbers both ways for your own situation first.
