I think a lot would have to do with the total value of the estate when the F.i.l. dies…
If it’s less than say, a million quid it’s simply a form filling/rubber stamp excersise. If the value of the estate is less than say, £750k, there won’t be any tax due, and HMRC won’t give a crap about figuring out who really paid for that 10k car…
… if the car was 1955 Mercedes-Benz 300 SLR Uhlenhaut Coupé, on the other hand…
Presumably the F.i.L has a will, this is part of why it’s important to get a will written properly…aside from who gets what, things like this can be straight forward rather than a mine field, and why it’s important to get advice rather a boiler-plate will.
And will prevent any in-fighting, such as party A saying, party B got a free 10k car ‘under the table’! not fair! etc, etc.