I know energenie do a relay and I also know items in the energenie range will connect to Nest, and it is likely they can be made to do what you want, however I have found the advertising rather useless when trying to find out exactly what they will do.
I bought a energy monitor and remote switch with the idea I could use the three remote controls I already have to switch it off/on and unlike the simple off/on sockets I could see on the phone if actually off or on. However when I go it found would not pair with the remote, it will only work with phone, PC or tablet using the hub.
So you could buy the relay and then find it will not pair with Nest. So only option is to ring the firms and ask, even then IFTTT changes every day, so it may have not worked yesterday but will work today, and you could be told it will not work when it will.
Big thing is the UFH electric or water? I had assumed electric, but of course could be connected to central heating boiler, the latter is low current and Nest, Hive, EvoHome, and Tado would all work fine, but with electric the current switched is a lot higher, and the problem is not fitting the relay, but fitting it for use by ordinary person. Every thing in a house has to be type tested.
So a relay like
this one is designed to fit inside a consumer unit,
so the likely way forward would be a consumer unit close to the thermostat with the relays inside, if you go to a hotel in Turkey they have boards with relays to turn the aircon off/on and as the tourist you have to pay extra to have it switched on. The problem to my mind is
a unit like this to mount the relay does not look good on the wall, it is where it can be mounted, maybe inside the existing consumer unit, it clearly can be done, but how would depend on the property.