That's why there's a trend to either *NOT* provide a 13A socket on a 32A circuit or provide a 45A circuit, which always allows up to 7.8KW 'flat-out' for the cooker (or 10.8KW if 45A with no 13A socket), and as the rings and oven are regulated, usually by 'bang-bang' controllers, its rare to have all rings and the oven element drawing current simultaneously except for a few minutes during warmup if you turn them all on at the same time.
If course if the customer isn't a penny squeezing idiot, a reputable electrician doing a new install or replacing the feed cable, will probably recommend a 45A circuit breaker and 10mm2 cable, especially if a double oven cooker is to be installed or if an isolator with auxiliary 13A socket is to be fitted. Use a long chain of contractors where you don't even get to talk to the electrician and haggle on the price and you'll usually get the bare code compliant minimum and occasionally a deficient bodge job with a bogus certificate.