Irrespective of any rules, I'd be concerned about your plans because panel heaters may not be designed for use in a potentially damp environment. In particular, they may have switches which can be operated by damp hands. (The only switches which should be used in bathrooms are those operated by pull-cords).
Forget about panel heaters. While they might be cheap to run, they're generally so low-powered that you'd need to leave them on all of the time. Get a proper bathroom fan heater, which can give you an instant blast of warm air when you actually need it:
http://www.argos.co.uk/webapp/wcs/stores/servl et/Search?storeId=10001&catalogId=1500000701&l angId=-1&searchTerms=4157904&x=0&y=0
To install the heater, you'll either need to use the services of an electrician who's registered with a 'competent person scheme' or you'll need to get approval from your local authority's Building Control department. See here:
http://www.eden.gov.uk/pdf/bc-odpm-electrical- safety-leaflet.pdf
Chris