Apple can't seem to fix MBA

My MBA 2013 has developed a weird problem where messages addresses always come up red, and it says the recipients address isn't registered and won't send or receive. It used to work but stopped 5 weeks ago.
In Yosemite it's also stopping FaceTime working, and handoff/continuity.
I tried obvious things like PRAM and clean install, several times in fact. Happens with other apple ids and if I target disk the laptop and boot my iMac off it it works.
I've been on to apple support uk with no joy for 4 weeks now, taking time to follow their engineers suggestions (of everything I tried before contacting them) and now the issue has been with engineers in Cupertino for a week with no reply.

When should I get annoyed and what should I be asking them to do?. Seems really silly and one advisor said they'd swap the machine but never said it since

That's my plan too I think, Have said will wait another week, but it seems the machine UUID has been blocked for some reason, and swapping would therefore work unless someone manages to unblock it.

Just frustrating that it's taking so long for a seemingly simple issue.