Boris Johnson seemed to have a vision of a rail bridge or tunnel to Northern Ireland, apparently with reinstated trains running from Carlisle to the Portpatrick area. Acquaintance with an OS map should have shown how far it was to go, and the roads that have been built on the old track bed. My grandfather used to take a boat train from Newcastle 100 years ago. Following in his footsteps to trace Irish roots I can confirm that improvements to the A75 are more important - and it's a very long way from the M6/A74(M).
A relatively cursory look at Boris Johnson's record will show a liturgy of grandiose infrastructure proposals that are wildly optimistic and have absolutely zero chance of every happening, yet because he gets elected people are forced to entertain him for a while.
Grain Airport, London Garden Bridge, Irish Bridge, Isle of Mann Tunnel, etc. The Emirates Air Line is perhaps the only one you can credit him with and even that is a financial basket case. HS2, Crossrail, London Bike Hire were all in development long before Johnson tried to claim any credit for them.
I'd be satisfied with the Stranrear line having a secure future.
You'll be needing about 15,000 to want to move to the far corner of SW Scotland then, or persuade P&O and Stena Line to move their ferry terminal back. If the UK could actually run integrated buses I think the line would look even shakier than it does. And even so it's mainly political emotions that are keeping the line open - it doesn't even generate enough traffic to justify investment in cheaper operating practices.