In certain circumstances, a heated towel rail is indeed capable of heating the whole bathroom, however this is not necessarily always the case.
There are many factors involved when deciding whether or not a heated towel rail will be enough to heat the whole room.Heated towel rails emit less heat than a radiator of equivalent size. The differing design of the two products means that towel rails have less surface area from which to emit heat.
Therefore, in order to ensure your bathroom is adequately heated, the heated towel rail you choose will have to be much larger in size than a radiator of the same heat output.
However, if your bathroom is quite large, you might struggle to find a towel rail that can heat your room to a comfortable temperature. If this is the case, you might find that it’s better to have a heated towel rail for convenience and a separate radiator that can handle the bulk of the heating.
If you’ve got a large bathroom then it’s highly probable that you’ve got enough wall space to install a towel rail and a radiator, so this may not be an issue.