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. The theme song for the dub has been uploaded to YouTube, and some episodes have been uplaoded, 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.
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.