Unvented multi zone system.

[Ric2013]

I do see your point about the pipework, but bear in mind, if you have a couple of lengths of 22 splitting off to a few rads, and the radiators are off, there will be no flow to heat up that section of pipework. On a two-pipe system, it isn't a loop as such.
Of course, a manifold system does have less water in it, but you may find that the heat loss from several lengths of narrow pipe is the same as that from one piece of 22. Unless you really only intend to heat one room and the house is very large? Of course, it is up to you as only you know the details and what is important to one person is not important to another, so you will ultimately choose what you feel is right.
That said, it sounds like you are giving this some thought, and I look forward to hearing of your progress.

 

[ShaunCorbs]

And a lot less cost you seen the price of decent 22 mm 2 port valves I wouldn't be far off thinking about 1k for the port valves and stats

 

[Ric2013]

This sort of thing, Shaun?:

 

[ShaunCorbs]

This sort of thing, Shaun?:
View attachment 32303

Yep better off using an underfloor heating manifold with heatmiser valves and stats

 

[Vee]

Or Evohome Ufh controller and you can use DTS92 wall stat to control the zone actuators. Or use DTS92 in conjunction with Radiator controller.