there is also a bit of a difference between taking a run to the shops and not working it in to your accounts that you've used some petrol, than not paying a thing for a year or letting the HMRC know your earning anying or paying ni either
Listen, I'm an ex-accountant (though not a tax accountant) and from a 'holier than thou' point of view, there is no difference between the two.
My accounts used to be audited once-per-month, with a 'biggy' once a year, and also five 'spot' checks thoughout the year. It was a fraud prevention thing; in case I was fiddling the books and buying luxury cars with the proceeds; so I know a fair bit about audits, auditing, and the process. Tax investigations will have a similar process.
But back to vehicle usage for private purposes.
It's wrong and illegal.
Oh, it's just a trip to the shops, or oh, I had to take my dog to the vet, or I took the van on a three-week-holiday to Italy; though I didn't claim the fuel; but yes, I haven't made any deductions for the road tax, vehicle insurance or capital allowance on the van!
Vehicle usage that you claim for should be for business purposes; and NOT private.
I don't care who you are or what you say, I bet if I looked through your books and bank statements, I could find 'anomolies'; whether you use an accountant or not.