It would be irresponsible of me not to mention at this point you don't really know what you're doing.
If you insist, you can use 2.5mm on a 32A breaker if it's a ring circuit. You could also use 4mm on the same breaker as a radial circuit. (probably easier for you if you know the difference )
Either can take a 2kw heater no probs.
Whatever you choose there are far more factors you need to take into account that i can't be bothered to run through here.
I urge you to consult a pro at least even if you don't contract him, if you want to keep things safe in your home.
oh yeh, and watch out for the part P police, what you're proposing needs to notified, tested and certified to keep legal and stop someone frying!
P.S. Try reading up in the Wiki if you're gonna try this solo.
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