Towel rail valve fitting.

I would like to put a thermostatic radiator valve in the bathroom towel rail.

I tried an existing tado valve I have with the standard fitting.

The metallic ring screws in, but the plastic bit is a bit loose. When I try to fit in the valve itself, it does not lock in as the pin sits too tall.

I tried the various valve plastic accessories in the tado box, without any success. Has anyone else faced a similar issue?