Neil I have exactly the same requirement. I went for Tado after a pre-sales conversation which led me to believe it would be no problem, only to discover when I’d spent the money (a lot, for 12 SRTs, 5 STs and an Extension Kit) that it isn’t really advisable due to lack of overheat protection and 6A limit.
I have an underfloor mat in each of four rooms that I wanted to control with A Tado ST to supplement the rads (controlled now with Tado SRTs).
Before I found your post I had concluded that what I needed was basically a very simple switch/relay that allows you to read the temp from the probe, set max temp (for heat protection) and which itself can be switched on by the Tado unit. I have found no such thing (tempted to see if it might be possible to make one but my knowledge is not sufficient to attempt it).
Did you get anywhere? Obviously it may be that Heatmiser would have been the better choice but we are where we are, right? Pleased with every other aspect of Tado so not inclined to send it all back.