Dunno if this is particularly relevant but it's a good trick anyway.
We had a large twin bar towel rail in the singing room but the
'management' wanted more heat. As said, towel radiators don't
chuck out much and are hideously expensive so after a lot of deep
thinking I had a rare brainwave. Since the rail was pretty big bore,
fit a radiator underneath the existing towel rail and plumb them in
series. Cheap, and worked a treat.

Best