Andyp wrote:I thought the ususal thing to do was add the bathroom radiator on some sort of continuous circuit linked to the hot water.
Before fully pumped systems it was common for the bathroom rad to be fed off the primary circulation pipes to the cylinder which were gravity fed, this meant any time the hot water was being heated so was the bathroom radiator. As heat up time of the cylinder was longer when gravity fed & generally wasn't controlled by a thermostat unlike a pumped system the rad had time to heat the bathroom.
There are still a few ways to independently heat a bathroom rad, as has been said you can fit an electric element. You can also install a zone valve & provide a circuit just for bathroom radiators, I've done this previously particularly where there has been several bathrooms.
The most unusual way I've seen (wasteful) was a brass towel warmer which was fed directly off the hot water cylinder, positioned above cylinder height on the bathroom wall the cylinder was the other side of the wall. Basically the towel warmer just gave off the heat of the water in the cylinder, though as the cylinder was gravity fed working off the boiler stat the heat loss wasn't noticed, though rather wasteful on gas.