Have you worked with underfloor heating systems before? The thermostatic controls work by maintain the floor at a set temperature by turning the element on and off like a normal thermostat so not all of the elements will turn on at the same time. They don't ever let the floor cool down completely as when the 'off' periods are active they merely maintain a lower temperature. So once the system is up and running you won't get it all on at the same time.
Use your training and good judgement to assess the diversity applicable and hey presto plenty of room for cooking, especially when most cookers come out at a lot less than 30 Amps after diversity.
Most underfloor heating companies will happily come out and design the install for you and you can get advice form them about applicable diversity.
And by the way you don't just tell the builder to install something else, YOU don't call the shots on what your client wants installed after all they will be paying the bill and having to live with it. You can advise them that a wet system might be a better alternative and explain your reasoning so that they can understand what is going on, don't forget they might be checking everything you tell them on forum like this!