I've bought the imported special edition from a UK seller, I can get the DLC from the US PSN store using Entropay and I use my US profile on the PS3 to play it and it all works great. I paid probably as much as it will cost when it finally comes out over here in september, and the DLC probably works out cheaper at $2 for each track and $6 for a track pack, I can image it will cost the same but in pounds on the UK store. Also there will be some Euro specific tracks in the UK version that will be released on the US store.
The only problem I will have is with warranty, so if anything breaks on the drum kit I shall have to replace it myself which isn't so bad, there are plenty of cheap fixes on auction sites and I can just wait until the UK release to use the warranty here.