you should always ask for a company receipt otherwise you are complicit
"There are no legal implications for either party to pay in cash for work, or offering a discount for paying in cash in order to avoid administration/banking charges.
However, this does not negate the trader’s obligations to declare the services and cash received to HMRC for TAX purposes. "
So I'd actually be commiting no crime.
http://www.thisismoney.co.uk/money/experts/article-4404786/Is-illegal-pay-cash-hand-handyman.html