I'm from the UK as well and only downloaded it a few weeks ago. You don't seem to be able to download it from the marketplace ingame (and I'm not sure if it appears on the UK marketplace available from the dashboard at all), so I had to get it from the following link:
http://marketplace.xbox.com/en-GB/Product/虫姫さまふたり/66acd000-77fe-1000-9115-d802435a07d7
After that,
be warned there is a serious bug in Black Label. Once you've got it installed it causes the game to randomly freeze in every game mode (NOT the usual deliberate slowdown), which is absolutely game breaking in a title as demanding as this.
I was playing the game straight off disc, not installed, and had the DLC on my hard disc and it caused the game to freeze up in this manner. I've seen people state that installing the game as well stops it from happening, but I've sure I've also seen people state the opposite. What I did was something suggested elsewhere: I moved the DLC from the hard drive to a USB stick I always have plugged into the XBox (if you've got one of the newer ones you could try sticking it on the internal 4GB flash storage). This causes it to take a second or two longer to load Black Label now, but fixes the freezes in all modes. Sadly it seems Cave will never fix this bug even though they definitely know about it.
