Why can't they get a feed from EGHH until then?
Ahh, but HH isn't run by NATS. You could ask the same question of the source at Jersey.
The reason that you might imagine this solution is kinda unlikely will depend on your degree of cynicism.
The real question might be that surely there must have been some SSR data within the NATS system that could have given some coverage in the Solent area - or was poor coverage considered worse than none?
BTW, nothing so technical as a microwave link would be needed, a telephone line suffices in most other cases.