There are special rules in the UK for electrical equipment to be installed
in bathrooms.
The requirements depend on where in the bathroom the equipment is placed
relative to the bath or shower, see BS 7671 section 601 which you should be
able to access at the local library.
There is also a requirement that the equipment should only be installed by
an Part P-certified electrician.

In general I would recommend that you only install a heater designed
specifically for bathroom use. Try doing a Goggle UK search there are lots
available.

I hope this helps

BillB

Yorkshire lad in exile.