As it's an infra-red heater I think you need a timer that is manual "on" and auto "off"
because it gets very hot it wouldn't be safe to have it automatically come on when unattended.
A suitable timer would be
www.tlc-direct.co.uk/Products/HOE30.html which allows you to turn it on for a timed period of up to two hours (this one has no fuse so would only be suitable for a dedicated circuit which is already fused elsewhere)
or this
www.tlc-direct.co.uk/Products/SMFBT4N.html which does have a fuse and can replace an existing FCU.
The makers have similar models with different time periods
UK has rules relating to electrical fittings in bathrooms. You can fit such a switch outside the bathroom door, or inside the bathroom provided it is far enough away from the footprint of a fixed bath or shower. Bathroom "zones" are not measured from WCs, bidets or washbasins. A qualified electrician will also know the regulations for distance, bonding, and RCD protection.
Many electrical fittings are allowed in a UK bathroom if they conform to the zones measured from fixed baths or showers. Even plug sockets are now permitted if they are three metres or more away, but the size of a normal UK bathroom means this is usually impossible so it is simpler, but less accurate, to say they are not permitted.
Many people think you aren't allowed an ordinary wall lightswitch, but, again this may be permissible, subject to measurement.
These rules do not apply to rooms that do not contain a fixed bath or shower and are therefore not bathrooms. A room with just a washbasin and/or WC is not a bathroom.