Smart TRVs - I don't understand

They have the ability to override the central thermostat and call for heat.
You'll need the rest of the ZigBee kit to make it work.

My Tado kit has a remote central thermostat(in a room with no stat on the rad), receiver, and an internet bridge. The rad stats talk to the receiver and override/call for heat via RF.
The internet bridge provides the ability to control from anywhere.

Nb the non trv rad isn't a necessity any more, but it's on an old cast iron rad so I don't want an ugly white box on it!

On our tado system you need the smart thermostat, it gives the signal to the boiler to provide heat when one or more of the smart trvs make the request.
If none of the smart trvs aren't asking for heat the boiler isn't running.

They're separate from your boiler/thermostat. I think the advantage of smart TRVs are that you can control the temp in any single room (assuming the heating is on).
You should never end up with the boiler pumping water and all rads off, as there should always be an open rad (usually this is the bathroom one). The key is not to have your thermostat and smart TRV in the same room (which would be pointless anyway)
EDIT: I'm referring to retrofitting smart TRVs without a full integrated system. Considering doing this myself to create an upstairs zone - normal TRVs are crap IMHO. My original Hive system is a bit basic

Screwfix have ZigBee based TCP (the brand) TRVs for £20 each, half price, but there’s no associated smart thermostat to go with them.

EDIT: I’m referring to retrofitting smart TRVs without a full integrated system

If you have a bypass radiator which is always on, then why can't you just control individual radiators with an app via wifi?

The call for heat is from leaving the boiler on continuous.

Obviously, you could, but the motivator for most people in getting a smart heating system is (at least in part) to save money! Also barely worth upgrading from manual TRVs if you then have to remember to turn on & off all the “smart” ones manually still.

Drayton wiser works this way - each TRV calls the boiler. You still need a Drayton main thermostat though

Drayton wiser works this way – each TRV calls the boiler.

you can’t just have smart TRVs, you need some method of calling for heat from the boiler. It is possible to DIY such a controller though.

Surely that's just an app controlled over WiFi though? Rather than a separate physical controller. I have a similar thing with lightbulbs. You can have a bridge with some (Hue etc.), or with others you can just control off your phone using an app and/or Google home
I have two rooms that only need heating 3 nights a week and an office I only need heating during the day if I'm WFH, so I'm thinking they may be worth getting for that (though I'm not sure whether they'd pay for themselves or not).
Edit: I could just manually alter the TRV in each room but it's a PITA.
Second edit: Ok, re-read and see your point. I was thinking more of a way of shutting down rooms remotely when not required (or on a timer) AND more accurately controlling the temp in then (than TRV numbered 1-6 which are crap)

I was thinking more of a way of shutting down rooms remotely when not required (or on a timer) AND more accurately controlling the temp in then (than TRV numbered 1-6 which are crap)

I want the ability to hear a single room, and not the hallways which is where the ballast rads are.

there is a TCP hub

I assumed it was just a ZigBee hub, are you saying it's actually a boiler controller too? I already have a Hue hub which is ZigBee.

Smart thermostats and smart TRVs seem to be sold as separate items that can be integrated, but surely both are essential?

No, a smart thermostat (and wireless relay box) bought in isolation is basically exactly the same as a traditional thermostat with the addition that it has WiFi connectivity so you can control it from a phone/tablet/laptop remotely by app or website. It'll just be one main thermostat/controller (usually in the hall) with the ability to set one schedule for the whole house, like a trad thermostat. You can then choose to add smart TRVs so that individual rooms can be controlled either on an individual schedule for that room or controlled remotely from phone/tablet/laptop.
I've only got my main rooms with smart TRVs, rooms like laundry and my kit room have trad TRVs that are always just set fairly low. In my case (evohome) once you start adding smart TRVs the thermostat in the main controller goes dumb (presumably so you don't have conflict).

I want the ability to hear a single room, and not the hallways which is where the ballast rads are.

What's a ballast rad? (I tried googling it, nothing came up). If you mean they are the open loop rads then they will come on every time one of your smart TRVs demands heat. So you might want to change your open loop rad to somewhere like the bathroom.

I assumed it was just a ZigBee hub,

If you mean they are the open loop rads then they will come on every time one of your smart TRVs demands heat. So you might want to change your open loop rad to somewhere like the bathroom.

Hallway makes sense in a single thermostat setup as your stat is normally in the hall too. It avoids a situation where the stat is calling for heat and a local dumb trv is stopping the rad from heating

That makes sense. Although very wasteful, as it meant the hallway was always the toastiest “room”!