Crusty .......... since towel rails normally draw less than 1 amp, they can be connected to a lighting circuit or a socket ring circuit via a fused spur with a 3amp fuse.
As OG has pointed out, Google "Bathroom Zones" to make sure it's sited properly.
Under the latest regs, bathrooms should be protected by an RCD. As this is an addition to the system, although you are allowed to do it yourself, it should follow the latest regs.
Do check your Consumer Unit for RCD protection.