A stand alone cooker will need a dedicated supply. The only one which will work off a 13 amp supply is a baby belling. You may have two supplies or one supply split and what will be needed to make this to a single supply I would not know. As to supply amps normally between 32 and 45 according to cooker and cable if supply too small it may trip the MCB in the consumer unit if you try to use too many rings but MCB matches the cable not the cooker you will not get a supply too big.
The supply should be tested of course especially with cookers which could have high earth currents.
Although most likely double pole switch I have not seen any regulation to say it should be double pole.
Because of the expense of test equipment plus Part P which is required for changes to wiring in a kitchen I would think you would be better getting an electrician to do the job.
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