I was looking into just this. I think the largest towel heating element I found was 600w. Most “normal rad” .5″ BSP, some 3/4_ BSP.
This would not fit a type 2 rad, but would a type 1 rad. Ideally you would want you rad to be plumbed top to bottom (either same end or opposite end) so that you could place the element low.
If you have a column radiator this would be better still as they have 1″ BSP or 1.5″ BSP fittings. There are are quite a few 1.5kw to 2.5kw elements once you get to this size fitting. They however are without thermostat control so you would have to rig up your own thermostat. This would probably be used as an interposing relay as most thermostats I have seen would not handle the current for a 1.5+kw element, but that’s easy.