On Mon, 3 May 2021 01:38:21 -0700 (PDT), Jedzi
wrote:

I've got an ancient, maybe 40 or 50 years old, switch for the immersion heater in my airing cupboard. It's a thin-ish one, mounted on the wooden door frame.

Increasingly, it gets hot in use and has started to smell slightly (I think it's a heated plastic type smell).

I want to replace it with a switch like this https://bit.ly/336ClVA , mounted in a steel back box, on the wall next to it. Am I allowed to do that?

I would say no. 240V x 20A = 4,800W. I believe some immersion
heaters can draw up to 6kW. Even if yours doesn't, someone might
upgrade or replace it.