The Backyardigans is a Canadian-American television series that aired on Nick Jr. from 2004 to 2010. Like Oswald, Wonder Pets!, Blue's Clues, and Yo Gabba Gabba!, it was redubbed for the UK market on Nick Jr. from 5th September 2005 until 2015, The theme song for the dub has been uploaded to YouTube, and some episodes have been uploaded, however you may need a VPN to view them
Lizzie Waterworth is known to have been the voice of Austin, as he sounds exactly like Horrid Henry on the CITV Channel from the namesake series. Waterworth, Janet James, Maria Darling, and Mia Murphy were the other voices of the Backyardigans. However, Alicia Keys (when she guest-starred in Mission to Mars) was not redubbed.
The show is on Amazon Prime UK, But it uses the American Dub.
UPDATE 08/13/2017: Amber The Fangirl has said that she has a The Backyardigans DVD, Cave Party, and it has the UK dub on it. The episodes will be uploaded to YouTube soon.
UPDATE 08/19/2019: BoyInCharge55 here. I noticed all of the videos containing the theme song on YouTube were blocked. The good thing, however: I found the British dub of Cops and Robots on Vimeo. From looking at the credits, the series was dubbed at Ten Pin Alley Limited in the United Kingdom.
UPDATE 12/13/2019: This is me again. I found a playlist with UK-dubbed Backyardigans videos on it. However, the first two videos on it got blocked. The rest of them are still available, though.
UPDATE 02/20/2021: AyanoxTaro here, I found a 2007 UK Backyardigans promo that was uploaded in 2019 https://www.youtube.com/watch?v=LCzxVL2PXiw
UPDATE 1/3/2021 : A Google drive folder with episodes on has been found https://drive.google.com/drive/u/0/mobile/folders/1rJK5XQEqmEDFRMyC68PO7FrUyhPseNJj
Update by CityOfTheNight: I found an eBay listing for a UK DVD, Surf's Up, I have it linked in the comments. If anybody can buy that before I ask my dad (I didn't have the courage to ask him because sometimes I feel weird asking for kids stuff and I'm not going there this weekend anyway) that would be much appreciated. Plus I would have to record it with my phone and I'll admit my hands are not steady when I record things, and I don't have a tripod. There would be better people to buy it than me if possible. The DVD ships worldwide, but it's region 2, so if you're not from the UK make sure you have a region free or Region 2 player.
Edit by CityOfTheNight: OK nevermind I think all of the episodes on that DVD have been found.