So, if you are the BBC, and you have a show that is your number 1 export, you will do all you can to sycronize overseas release dates with original air dates. That way you can cut down on illegial downloading and minimize spoiler-caused arguments between foriegn and native fans.*
If you have spinoffs of said show, attempt to market them under the same principles: "You like show X? Well, Y and Z are set in the same world, with some of the same characters." In this case, show Y (a more 'mature' spinoff) ended up being a cross-pond production and released on this side of the Atlantic first. But show Z, a children's show(as in, the main cast are teens) is not broadcast here, and the gap between British and American DVD releases so far has been -1 month (ie, one month before), one day, two months, and one month. However, the first three series were put on iTunes, while the latter two were not.
All this to say...
I WANNA WATCH Curse of Clyde Langer part 2! AND I CAN'T YET! gaaaaaaaaaaaaaaaaaaaaaar!
*Which, in general, they did well with this year.