I think this might help clear up the original question - 1985/6 vintage network routings
http://txlib.mb21.co.uk/main.php?g2_view=core.DownloadItem&g2_itemId=4001
So firstly, it seems that English regions didn't do opt outs on BBC2 until late 1986 so until then the circuits only went to the "big" centres with switching centres (eg Birmingham, Manchester, Bristol plus the nations) not the "smaller" centres (I think they are called Island sites in BBC parlance?)
Page 12 shows how the network got from Manchester to BT Carlisle, from where there were seperate BBC2 circuits to Belfast, Glasgow, Pontop Pike and Caldbeck.
At BT Carlisle, there were switching facilities under the control of BBC Manchester shown on page 32. On the BBC2 side, there was the main distribution circuit from Manchester and a "new circuit" which could be fed to various destinations.
So, to get a feed from London to Glasgow via the distribution circuit, it seems that the arrangement would be:
Manchester put test card on BBC2 distribution to all destinations including Carlisle
Feed incoming network into the "new circuit" to Carlisle
Make sure Glasgow has opted out
Switch New circuit onto Glasgow circuit at Carlisle
Therefore the Test Card G appearing in Northern Ireland would have originated in Manchester.
Some other interesting points from the documents - in that era some of the regional centres were only connected to the transmitters at opt out times, so the circuits were available for contributions. Others, like Leeds, could be bypassed by BT so the feed from Manchester could be sent directly to the transmitters, freeing up the circuits, so presumably if there was material being fed to Leeds via the distribution circuits, that's how they would have done it.
That Network Routings book takes me back!
I was wrong about CP sitting on the main network feeds from LO SWC, it appears it was the other way around with the main feed going to BM, BS and ROW and CP getting the reserve normally.
That does solve the mystery of the Carlisle switcher, I wonder when the "New MR-CE" vision circuit appeared though.
If this was dated December 1986, note that there is no BT circuit number on the diagram (W12XXX), so presumably this only went into service after that time.
As for the point about the "island" sites such as Leeds, I'm not sure that is correct. BT bypassing anywhere would not be a clean switch. It could be as basic as pulling a U link out and sticking a patch cord in (rather than use a matrix, which itself could have faults). It would not be something done unless there was a fault condition on the BBC networks, but ITV worked very differently. Unless it was stated on these diagrams, I don't expect there to be a switch as such. (I am prepared to be proved wrong on that!)
Norwich on the other hand had to switch Tacolneston remotely to a different feed, as well as send Look East down the contribution circuit to Birmingham which passed through Morborne Hill and then BT Peterborough, where another remote switch was. It demonstrates how different the regions had to be with opting out with each area having quite a mix of BT feeds, RBL and BBC provided radio links and remote switches dotted around the place!
