That really doesn't matter as HMRC will just estimate the value of the car. They do the same thing for income tax in "payment in kind". If your employer gives perks that are not money then the value of them is estimated for tax calculation.

So you would still be liable for CGT in the case of buying something like a car with Bitcoin. The one thing you didn't mention about this is that CGT is only due on the gain not the full value of the Bitcoin, so if you bought Bitcoin recently then the gain would be far less than if you were holding it from a year ago or more.