Nest Gen 3 has two sets of contacts and a variable voltage connection if using OpenTherm, it is unlikely any reason why Nest can't control DHW independent to CH, however with my house the plumbing will not allow independent control, and my plumber advised me to add the motorised valves to turn off the DHW would likely stress the boiler.
As to if true or not I have not looked into, however I can't see why I would not want DHW, with a 20 minute burn time 3 times a week it keeps the DHW hot, it does not cool fast so there is simply no point stopping it heating up with the central heating.
And the wiring involved to wire a thermostat on the storage tank to a motorised valve at the boiler would involve lifting two floors, just not worth the effort.
In theory changing from Plan C to Plan Y could change it and allow temperature control of DHW, but I would likely be looking at a £500 bill or more, so simply not worth changing.