What do I need to buy to convert towel rail to electric?

This can be done, usually you would buy a specific, electric only heated towel rail, a little easier and convenient, ask Victoria Plumb to swap it for you. If not you would need a heating element/thermostat, and also a specific liquid to fill the towel rail. Fernox make the light oil to fill the radiator, this can be purchased at most plumbers merchants; fit your heating element as per instructions in either of the bottom valve inlets. Fit your plugs/caps into two of the other outlets and using a funnel fill the radiator with the oil into the last open outlet, leaving about a 2cm gap at the top for a little expansion, replace the final plug, now ready to use

You need to buy the oil from a plumbers merchant and fill it with that and fit the right size element that would also be available at the plumbers merchant and that will change it to complete electric radiator.

To be totally honest it will be far easier to exchange towel rail for a dedicated oil filled electric towel rail, but if you want it on the central heating it really shouldn't be to hard to do unless there no access to existing heating pipes.