You could do as you suggest, but I don't think it would be acceptable in a bathroom to have an "ordinary" switch, at least not in the UK.
Nothing to stop you from mounting a ceiling pull cord shower switch and routing the connection to the heater via said switch, though.....
Something like this:
https://www.screwfix.com/p/british-g...E&gclsrc=aw.ds
with one of these:
https://www.screwfix.com/p/british-g...mm-white/83819
(Other makes/types/sources are available)