Realisitically, it's unlikely that a loft extension will ever require heat when the first floor doesn't. In my experience, lofts are usually overheated places even when the heat's off, because hot air rises up from the rest of the house.
Quite apart from the issue of the valves, there's also a potential problem with the pipes. If your exisitng pump and 3-port are already on the first floor beside the cylinder - should be no major problem. But if they're on the ground floor, you'd have to run a separate flow and return all the way up to the loft.
Why not stick with your existing 3-port, carry on using it to fire the boiler as now, and add a 2-port valve just downstream from it to serve the loft. Then use a room stat actually in the loft to open the valve as required, without any connections to the Grey or Orange wires. Then, the main house rads will work independently from the loft but the loft rads will only work when there is ALREADY a call for heat from the main room stat (which will operate the 3-port and fire the boiler). I did it this way in a house where it was virtually impossible to get a new pipe run from 2nd floor to basement, and as far as I know it works well enough that no-one has ever noticed the 'bodge'.
If you have TRV's fitted as well, it will be even less noticeable.