Hi Simon D.
A unit of electricity is 1000 watts per hour. That is what the electricity supplier uses to work out your bill.
If you have a look at your last statement it will tell you what price your supplier is charging you.
E.G. if you use 3000 watts per hour @ say 10p per unit, then you would pay 30 pence per hour, i.e. 3 x 10p, and so on. Your towel rail at 10 pence a unit would cost 7.5 pence per hour.
Working on this theory you can work out hou much any electrical appliance costs to run.
Not knowing what your supplier charges per unit, I cannot tell you what you would be charged for your heated towel rail, but I am sure that you can now work this out for yourself.
Regards