Times when one rival TV channel advertises on another

Yes, I remember seeing a news report on it somewhere once. TVS said they were still going to stick to their guns and refuse to broadcast ads for Sky and BSB despite the ruling (though whether they actually did is another question).

I think they caved in eventually, because I remember seeing the ads. Of course, during 1992 they were quite happy to sell news footage and reports to Sky News !

I'm not sure if this was always the case, though I think relations have mellowed somewhat since BT Sport's launch!

I had to trawl the archives of when I was able to capture, but here's one such example. Not the best quality, but you get the idea...

C4 News is the only thing I ever remember Central promoting.

Dennis Potter's last two plays Karaoke and Dark Lazarus which were interlinked were co-produced by BBC 2 and Channel 4 at Potter's suggestion and I believe were referred to by each other as this article from Wikipedia shows:

https://en.wikipedia.org/wiki/Karaoke_(TV_series)

A similar event happened in Australia but under different circumstances. Following the Boxing Day 2004 Tsunami that devastated a large swathe of SE Asia the three main commercial networks in Australia, Channel 7, Channel 9 and Channel 10 simulcasted a charity event in early 2005 to raise funds.

The event which was suggested by Adam Boland who was then the producer of Channel 7's "Sunrise" was split between a concert at the Sydney Opera House and a telethon in Melbourne. It raised over $20 Million (Australian).

From Wikipedia:
Australia's three major commercial television networks, the Seven Network, the Nine Network and Network Ten took the unprecedented step of pooling resources to organise a special concert telethon which aired on 8 January. Simultaneously broadcast on all three networks and in most capitals on the Triple M radio network, the telethon was an extraordinary display of solidarity and co-operation in the usually fiercely competitive commercial television and radio markets (see Australia Unites: Reach Out To Asia). As of the end of the broadcast, the figure raised/pledged was A$15,198,349.53, however phone lines remained open and the final tally totalled at over A$20 million. The overall corporate response has been criticised as inadequate, particularly considering bumper profits. The Australian public raised over at least A$190 million (US$143.37 million). As a sign of respect to the victims of the disaster, New Year's Eve celebrations around the country were either toned down or cancelled. Most of those celebrations that did proceed organised collections for charities in an attempt to avoid criticism. Australia's largest New Year's celebration, in Sydney, alone raised over A$1.1 million for the cause.[79]

There was some charity thing fairly recently - last few years - that was BBC and C4 cross promoted.

Happens all the time in Wales with S4C of course

Sky more recently was allowed to turn down specific BT Sport adverts, but not the more generic ones.

Detail of the rules and the history are carried in the OFCOM judgment https://www.ofcom.org.uk/__data/assets/pdf_file/0020/51626/bt-sport.pdf

Dennis Potter's last two series, that were shown together on BBC 1 and C4 were cross promoted. As were a series of Tosca opera broadcasts that BBC 2 and C4 screened in turn

Yes, Tosca was in 1992, the three acts were all staged at the specific locations at the exact times mentioned in the opera itself, and BBC2 showed them all live, including one at 6am, before C4 showed the complete opera that evening.
https://genome.ch.bbc.co.uk/schedules/bbctwo/england/1992-07-11#at-11.00
I remember Placido Domingo tripped up at one point and nearly went flying across the stage, I don't know if they edited that out for the C4 repeat.

Of course, at the end of the 2012 Olympics the Beeb talked about the Paralympics and specifically said they were on 5 Live and Channel 4.

After lTV Digital collapsed in 2002 the BBC had this:
https://www.youtube.com/watch?time_continue=2&v=z027Ojgyqvw&feature=emb_logo

Karaoke was shown on BBC1 on Sunday nights with a repeat on Channel 4 the next day, then Cold Lazarus was on Channel 4 on Sundays with a Monday repeat on BBC1. Both channels promoted both slots, you can see an example of the BBC trail here:




Think they showed Channel 4's Superhumans promo at least once as well.