cash, cheque whatever its the same price.
your not vat registered so it doesn;t matter.
People only do this so they can charge less the VAT and pocket the cash - which seems pointless if your not VAT regged.
However always claim any money the IR are commissioned based and will track WC's and all other cash type business's to investigate their monetary dealings.. not worth the hassle.
I know a lady that works for customs and excise and she is always asking trades about cash deals, less tax etc.. people will happily talk to her not knowing who she is or who she works for.. :

trust no one, keep quiet!