Watching back a recording of it, it seems they had to relocate to Bournemouth from the Isle of Wight due to flooding, and Live and Deadly's Naomi Wilkinson presented in place of Helen Skelton.
That was always going to be the case, when they announced the tour the press release explicity said Helen would be on holiday for this one, presumably in case you were only going for her. She chose the right one to miss. They announced they were moving to Bournemouth on Thursday's show, I don't know when they made the decision. Prank Patrol is clearly the CBBC equivalent of Coast and you could tell how old it was by the fact Barney looked about ten years younger.
I recorded all this so didn't know what happened after 9.30, but it was a bit rubbish that after Prank Patrol they went straight back to the schedule without any further apologies, presumably the price you pay for using pre-recorded announcements on Saturday mornings. I don't think the one in a tent the other week was anything to do with the weather, mind, I think that was just down to the venue.
Of course when Blue Peter did their summer tour a decade or so ago, they had to record one show a few hours before transmission as it was chucking it down and getting worse, and Blue almost pulled out until the production team said they'd announce on air they'd refused to perform because it was raining. And the first ever time they did an entire Blue Peter on location, in Carrickfergus in 1989, they had to transmit the dress rehearsal because a strike started at five o'clock.