"If UK PLC loses its ballistic missile capability, it loses its strategic nuclear capability. That is the bottom line."
And what is your point?
Theatre launched air-breathing missiles are exactly that - theatre launched. There has to be a launch platform in theatre. Ballistic missiles have a much larger range and much faster response time and are truly strategic. The rapid deployment is a large part of the deterrent. If something ballistic is thrown at you, there is little you can do against it other than watch it in the descent phase, even if you pick it up somehow! Ballistic missiles travel much(!) faster than a GR4 (or equivalent) with a standoff munition.
The dodgy dossier (mk 2) made much of Saddam's supposed capability to hit Cyprus with a BM.
In any case, the footprint required to stage a strike would not be cheap! Airstrip, AAR, force protection, SEAD, ISTAR etc! This would also depend on getting enough notice to deploy forces or having them in theatre already - garrisons worldwide at what cost? Or an expensive naval fleet! Even a sub-launched TLABM would require to the sub to be in position and that takes time.
Trident is over-expensive - agreed - and a cheaper version using the same concept would be very desirable. TLABMs ain't it!