good question
I hope for some good answers.
I have also considered the situation, but from a drivers view point.
To answer the last question first: with out TCAC, I now feel naked. I have had intermittenat TCAS failures in last few weeks, yes, I know this is a major problem, and should be reported, but..................
You guys in ATC have no knowledge of our status as far as TCAS goes, and must assume that it is fully functional, unless S/E.
We have twin ATC, so should never be without. BUT lets assume that we have no ATC, in todays busy skies, I would be tempeted to say that we must land in the FIR that is alerted to the problem. FL90 transditting the London TMA, should be banned, and a reroute North, with Lon Mil, would be my suggestion, plus a F15, as wing man.
Or "clear to leave controlled airspace".
only a thought...glf