When did it become common for pres desks to have a rehearsal mode, so it was technically still in circuit but in reality just passing a single source through so the next item could be rehearsed?
I know such a thing has available in radio for decades, but not sure when TV adopted it
Not sure. The analogue Pres mixer I was using in the early 2000s (and which had a 'memory rehearse' mode) was installed in the early 1990s I think. I'm sure MMcG will be able to tell us the exact year. Not sure if the previous desk had a memory or rehearsal mode or not (I think it was installed around 1985?).
BBC NI's new early 90s con suite went live in 1992. For the first time, the local announcer/director could now mix between a local source and the network feed (network and local sources were finally properly synced up, allowing for clean transitions). The desk also allowed for wipe transitions. The day that it went live - a Saturday - announcer/director David Olver made use of some of those new transition effects. From memory, there were still some minor local/network sync issues initially, but these were quickly ironed out.
The memory capability came about as a result of a need to make operation of the desk as easy as possible for the con directors/announcers in the nations, who had many operational items to contend with, before, during and after junctions. It was possible to programme the events and transitions in advance, as far as I'm aware.
The previous suite was installed in late-1984 as far as I can recall. It was part of a more significant facilities upgrade, which included the construction of Studio B. From a pres perspective, the new con facility also resulted in BBC Northern Ireland being the first of the BBC regions (national and English regions) to use an electronically generated on-air clock on BBC One. BBC NI was the only region to have an electronic channel clock during the 1981 - 1985 era.
I had a hoke about for a high-level spec of the 1984 suite: a comprehensive Cox special mixer, Rank Cintel MK7 slide scanner, Aston 3 character generator and separate network and clock logos; switching, as in the whole complex, is handled by an NTP router. The continuity has facilities to opt-out of either network and incorporates a special network mimic diagram.