This is definitely sad news! I remember my dad watched this religiously - and by default I would too, though I haven't watched in a few years either except for the crossover episodes with Casualty. Connie Beauchamp remains one of my favourite ever characters from the show and is the reason I started to watch Casualty.
But there were a fair bit of character crossovers between the two. Without it being a major crossover event. In early series of Holby City, some episodes featured scenes downstairs in the Emergency Department, so you would sometimes see appearances by Casualty cast members. Like seeing Ric Griffin downstairs in the ED with Casualty’s Dr Simon Kaminski (who exploded on his wedding day). Or seeing Alex Adams downstairs with Casualty’s Dr Lara Stone (whose cheating fiancé, Simon Kaminski, exploded and expired on their wedding day, thanks to Maxwell Caulfield). Or the paramedics from Casualty were seen attending a shout on Holby City.
Nick Jordan and Julie Fitzjohn debuted in Casualty. Nick was in two episodes of Casualty before Holby City. Julie featured in about four Casualty episodes. Charlie Fairhead and Chloe Hill from Casualty each did four Holby City episodes. Various paramedics from Casualty turned up in Holby City. Finlay Newton from Casualty was in about four Holby City episodes. Comfort, Greg, Dixie, Iain and Jez from Casualty were also seen in Holby City. Duffy from Casualty appeared in Holby City series two. Lara Stone from Casualty was in two episodes of Holby City. Mubbs from maternity in Holby City was in a few episodes of Casualty. Connie Beauchamp came downstairs for an episode of Casualty, when she was in Holby City. And went upstairs for episodes of Holby City when she was in Casualty. Henrik Hanssen from Holby City was in five episodes of series 25 of Casualty alone, for some major clashes with Niccolo ‘Nick’ Jordan. Donna from Holby City did a shift in Casualty. David from Casualty did two consecutive episodes of Holby City. Elliot Hope from Holby City was in episodes of Casualty and an old friend of Charlie’s. Anton Meyer from Holby appeared in Casualty. Ric Griffin from Holby City did two consecutive Casualty episodes. Simon Kaminski (who later expired after exploding on his wedding day), Dylan, Connie Beauchamp, Nick Jordan, Simon, Max, Ethan, Big Mac, Noel, Tony (whose brother exploded on his wedding day), Ethan (whose fiancée got hit by a train on their wedding day), Robyn, Marty from Casualty have been in Holby City. While Ric, Michael, Fletch, Connie Beauchamp, Jac, Jayne, Max McGerry, Guy, Lofty, Ben, Henrik Hanssen are among those from Holby City who made crossover appearances on Casualty.