Another regular BBC/ITV simulcast of the 70s/80s I seem to remember were the FA Cup Draws. I only ever remember them being hosted by the BBC, in the Grandstand studio, and World of Sport opted in?
I don't think the FA Cup draws were ever televised in that era, they were always on the radio alone. They used to do the League Cup draw on TV, but I think ITV had them alone - certainly Brian Moore told the story of how they did one once on On The Ball and five minutes before they went live, they dropped the balls all over the studio floor and only found the last one with seconds to go. So in the next round they pre-recorded it, and when Brian introduced it, they cut to the caption with the completed draw.
I know there was an England vs Scotland match in the seventies, in the days both channels simulcast it as one of the few live football matches a season, when ITV lost pictures and simulcast the Beeb's for a few minutes while they got them back. The Beeb deliberately didn't show any of their graphics while they were simulcasting, and both broadcasters made their pictures available to each other in case of this exact situation.
I think the Queen's Speech was another programme played out by the Beeb which ITV opted into, at least for a bit.