If you want this done properly then the 22mm backbone should be extended out and then branched out in 15mm to each rad. If you are going to screed then one way would be to lay ducting for the pipes to run in, or a channel with a removable cover at each joint. There is also nothing stopping the plumber from running 22mm down to a manifold outwith the screed and then running continuous 15mm pipe out to each rad from there, balancing then becomes key but that should be an installers bread and butter. Many ways to do this properly. As an addition to this, as long as copper is used and that copper is soldered properly and pressure tested then it for all intents and purposes it is then considered to be a single pipe for these purposes and can be encased in screed. Before the pipe is then covered in screed it must be protected, either wrapped or sleeved.
Any reason you aren't putting in UFH? It seems like a bit of a no brainer?
As far as WRAS 2.7 is concerned, then that specifically covers mechanical fittings that become inaccessible - looking at the scope of the The Water Supply (Water Fittings) Regulations 1999 - specifically section 1.3 the regs seem to state that they are only applicable to pipework that is or could be connected to water supplied to the undertaker/transporter (mains water). Obviously CH pipework isn't connected to the mains. I think the jury is still out on this and how it is to be properly interpreted as some read it as any pipework in a property that has mains water. Does that mean if I install a rainwater recovery system and run it to all my toilets on a complete closed circuit then the regs apply, I don't think they would care about that TBH. That's the problem with regs, they can be really woolly sometimes.
