16mm T+E for Loft Extension, (en- suit shower. )

[Maddison]

There's no such thing as "pushing the boundaries" in a compliant circuit. Either a cable can carry that amount of current or it can't. I think you will find that stated values err on the conservative side too. I have seen showers fed in 2.5mm cable that have not caught fire.
I always sleep easy as I do my calcs and install to them.
Not sure where your lack of confidence in calcs stems from, but hey, if you like overspeccing that's your preference, nothing against you either.
I have never installed more than a 6mm to any cooker or hob, and only last week saw a new induction hob with a 13A plug top as supplied.

I worked with with another sparky a few years back and he had to install this range cooker. Him and the boss came to the conclusion that a 16mm was what was needed. Couple days later I show up like have you lot never heard to diversity.... F**kinghell. They ended up having to put in a 3 phase rotary isolator just to be able to terminate the cables.. I think there's going over the top and just not thinking he was a good sparky in all fairness