If you paid in actual cash this may trigger a 'Where did this come from?', at which point you can show your receipt for the cash withdrawal,
Any large transaction involving anyone who needs to follow Know Your Customer and Anti-Money Laundering rules - and any property purchase is going to do that - will need to do 'source of funds' to document that they asked WTF the money came from.
"They turned up with a big bag of cash" is not going to be seen as an acceptable answer for them to write down even if you can show the receipts from the cash machine, but then neither is "they transferred it from another account".
'I've been working as a {insert whatever is on your self-assessment form}, here are my copies of the form' would / should be fine, and that's the level of answer a solicitor etc is going to want to see.
(About five years ago, a Bank of Mum bought their offspring a flat. Selling the old house in London made enough to buy one somewhere else, and there was enough left over to get a flat - again outside London - without a mortgage a few years later. The lawyers needed to see proof of the sale before they would take the money. No proof, no purchase.)