.... Design current for this oven is 13 amp .... Therefore closest breaker size but higher than 13 amps is 16 amp. I will be using a 16 amp MCB which is within the protection range of the RCD
Minimum cable size for this initially appears to be 2.5mm T&E based on reference method 103. However, when I look at the voltage drop calculations and tables, I think this requires a minimum of 4mm T&E. This seems to be adequate up to a length of 66 metres. I have a 12 metre distance.
That said, I think the cable rating needs to be higher than the breaker rating? So, perhaps it needs to be 6mm T&E?
I think you are probably getting too concerned about voltage drop.
For a start, there is essentially no requirement regarding VD in the regs, beyond the fact that voltage drop should not be so great as to "impair the safe functioning of the equipment" - and I think you would struggle to think of any equipment whose "safe functioned" would be impaired by its supply voltage being too low. The 5% 'maximum' VD (for 'power' circuits) is merely a
recommendation in an ('informative') Appendix of the regs.
In addition to there being no 'safety' issue, there is not even much of a 'functional' issue. Nearly all' high power' loads are thermostatically-controlled producers of heat (ovens, hobs, immersions, space heaters etc.). Even if their supply voltage is very low (due to large VD), they will still achieve the same desired temperature, the only difference being that the thermostat will cause them to be 'on' for longer periods, so gthey would take a little longer to initially 'heat up'. For the eco- or cost-conscious, higher VD means more energy 'wasted' in heating the cables - but even that may not always be truly 'wasted' if it results in the house being heated (at times when house heating is required).
I think most people would probably say that it would be plain silly to consider even 4mm², let alone 6mm² cable to supply a 13A load - unless, perhaps, the cable were incredibly long.
In any event, the 5% 'maximum recommended VD' can be argued to be rather silly, since it relates only to the VD within the installation, regardless of what the supply voltage is at the origin of the installation. For an installation with a supply voltage of 253V, even a 10% (of 230V) VD within the installation will only reduce the voltage at the load to 230V, which is considerably greater than the minimum permitted voltage at the origin (216.2V) and even more greater than would be the ('permitted') voltage (205.39V) at the load if the vo9ltage at origin wer 216.2 V and the VD within the installation 5% (of 230V).
As so often, I think that common sense is an important consideration.
Kind Regards, John