6mm installed in a wall is rated at 47A.
As Aandyg1966 implies, the mcb must be less than the current rating of the cable whilst the load has to be less than or equal to the current rating of the MCB. In essence the cable must be able to carry the load and the mcb break before the cable acts as a fuse and burns.
To complicate matters you can apply “diversity” to the load as the oven will not be pulling full whack all the time.
The problem you have is the hob is only going to have a small cable to it as it only requires a small current to fire the gas ignitor. As a consequence the cable to the hob will be undersized (probably only 1.5mm at most) for the mcb you’ll be fitting for the oven.
Now, the regs allow this if the cable is less than 3m, the load is fixed (ie can’t vary) and the cable size is such that the prospective fault current (PFC) can be carried until the MCB clears the fault.
Optimally you’d need to check the PFC, work out the mimimum cross sectional area of the cable from the adiabatic equation and see if it is big enough.
Alternatively feed the hob from the kitchen ring fused down via a fused spur to 3A.
Rich.