The MCB size is determined by the size of the cable you use. If the cable is 6mm all the way from the MCB to the cooker then you can leave the MCB alone. If you downsize the cable the MCB should always be sized such that its rated current is less than current carrying capacity of the cable. So, if you downsize to 2.5mm cable the MCB should be less than or equal to 27 amps (the rating of 2.5FTE), so a 16A or 20A would be ok. Can’t be less than 16A though as the load is 15 amps.
So, to summarize – a 16A MCB is ok and you can go as small as 2.5mm on the cable size.
I should add that cables run in insulation have a lower maximum current rating than those in a concrete wall so this may influence what is used. Difficult to tell without seeing it “in the flesh”. For this reason I’d probably use as big a cable as can fit. 4mm better than 2.5mm; 6mm better still.
Rich.