See my other post from today regarding ITV HD regions.
All transmitters are fed with the muxes fully assembled at code and mux centres at centralised locations.
In Fremont Point's case, the BBC Mux (that contains BBC 1 Channel Islands) is fed from the BBC C/M centres, at two fairly secret locations, on the UK mainland. The BBC Jersey output, is indeed fed via Plymouth to those centres. There is no direct connection between the Jersey studio and Fremont Point.
The fibre problems led to a loss of that incoming feed from the mainland (it may have also affected the link to Plymouth) so Fremont switched to RBS (1970s expression used by the BBC and IBA Re Broadcast Standby).
All the main UK DTT
transmitters have a satellite 'RBS' transponder that carries core national services, to maintain PSB 1 and 3 in case of fibre (etc) feed failures.
Only national (Eng/Scot/NI/Wales) BBC versions are carried, not the English regions.
So in the CI case, BBC 1 would switch to the generic English sustaining feed.