I've known of situations where people have tried this. The way to do it legally is to put a clause in your contract that title to the goods supplied as part of the installation does not transfer to the customer (the kitchen fitter) until fully paid for. There's the problem, of course - the goods are installed in the end customer's property - not with the kitchen fitter. And 95% of such contracts aren't written - they are verbal deals between mates.
Frankly the better bet is to simply refuse to issue the Part P certificate - that's a real gotcha because any self-certifiying electrician is likely to refuse to certify another's work - especially when it is all covered over. Since the value of the fittings is (by and large) pretty small is relation to the value of the labour, it is not such a bad 'gotcha'. Far better than the equivalent 'gotcha' a plumber might expect.