Not sure what you mean by "a large amount of cash", if it was over £10,000 then the car dealer is obliged by law to inform the authorities due to money laundering regulations.
Yes it was slightly over 10k and the rest i put on my credit card, but it was 3 years ago . They weren't used to taking large amounts of cash and i had to meet them at their bank so that could pay it in there and then,
but i wanted some modifications done to the car so, i was happy to walk away if they wouldn't accept cash as i didn't want to have cash lying around ( to easy to spend it on crap )
I gave them my drivers licence
I wasn't worried about the tax man because I could, even now prove where the money came from (small inheritance and from selling previous car ) so no worries on that score
But i agree that its hard to get rid of cash in some situations