Not entirely. I issue EICRs, EICs etc. and send them with the invoice (electronically or in paper form) but I don't issue the Part P certificate until I've been paid.
I'm with NAPIT and they agreed that the customer paying could be considered as the last part of the job/contract and so the job hasn't been completed until they've paid. Issuing the Part P certificate within 14 days of the customer paying, was therefore then acceptable no matter how long it was after I'd actually finished on site.
I've been stung so many times now that if I'm going to be on a job that's more than a couple of days work, or has lots of up front materials to buy, then my Terms and Conditions are that the customers pays me 50% on commencement of work and 50% on completion, or if it's dragging on due to eg waiting for other trades, 50% of the diminishing, outstanding balance each time I issue a stage invoice.
When I give them my quote they also get the T&Cs and a simple 1 page contract Trading Standards helped me draw up which basically says I'll do what's described in the quote/estimate, and they'll pay me the amount, we both sign it and have a copy. There's a section for any variations that may be required.
If having had the EICR etc they don't pay I just go straight to Money Claim Online (MCOL), add the cost of issuing a summons, interest and any other costs and MCOL send them the summons. Takes me about 20 minutes to do so. I can't be bothered to waste any more of my time than I have to chasing customers and this works rather well.