Since the 1st of June, Boomerang converted it's screen aspect ratio from 4:3 to 16:9 letterbox widescreen. All shows originally made in widescreen are now broadcast in their full frame, while other shows stayed in their original 4:3 format.
Then from June 10th, satellite tests had been made for a HD version of the channel, as mentioned on Digitalspy. http://forums.digitalspy.co.uk/showthread.php?t=2077694&page=3
As of June 24th, Boomerang HD has now officially launched in the UK. https://twitter.com/a516digital/status/613667087769321472
It is currently available only on Sky HD, on channel 603, moving it's SD feed to channel 636. (these changes are not affected on the standard definition service)
Boomerang HD, SD and +1 are also in full frame aspect ratio.
Other providers have yet to announce when Boomerang HD will be available to them. It'll most likely happen around 2016/17.