Your solicitor friend is partly right. True that they aren't very concerned about small ordinary chattels of a personal nature but, as you may realise, if someone has a £200,000 watch, an Imperial Faberge snuff box or a £6 million yacht , saying that it was a personal chattel and the Revenue shouldn't worry when and how it was given doesn't get the executors very far! The Revenue are keen to know what gifts of any kind were made in the 7 years prior to death. The Bentley is likely to be something of interest. They will try to include it in the estate, if they can. If they do claim it to tax, which, almost needless to say, they will try (if only as a bargaining chip), you'll have to prove them wrong.
Still, your solicitor friend might draw up a deed for you, for free (friends, even lawyers, do things free!), or show you the form: WHEREAS.........NOW this deed WITNESSETH...etc Not strictly necesaary in that old, legal, form. Any document signed and witnessed and saying the same will do (I am so old, sometimes: I haven't seen one in that form for such a thing in years)