I don't do it. I don't mind if it is (say) a tenner to the window cleaner, but if it's a few hundred pounds then I always pay by bank transfer, and click 'paying for a service' on the dropdown list.
I realise that lots of tiny payments add up, but it can seem like too much hassle to pay via the bank if I have a £10 note in my purse. Also, it's not always going to be contributing to the black economy - my window cleaner keeps a ledger where he records payments, so it's perfectly possible that he reports them to the tax man. He may not, of course.
When it comes to house repairs or renovations, I like the security of being able to prove that I've paid, in case of problems down the line. I also think that we should all pay tax, so don't want to collude in letting people get out of it.