I noticed in Screwfix they have normal radiator towel rails used in a
CH system. But they have electric immersion insertion heaters which
go in the bottom rad valve tapping to use in summer as well.

I have a few questions.

Do these towel rail immersion heaters have a tee piece for the rad
valves?

If I want the towel rail to be electric only and not connect to the
CH, I assume I fill the towel rail with water. What happens if the
towel rail immersion thermostat fails? Will the water boil and the
rail blow up?