Maybe it's different for the UK, because right now I see DI with 33% off, with Mexico as current location. GG also has the same discount but a smaller base price, which makes it a bit cheaper, and it activates on Steam for those who find it of the utmost importance. On the other hand, the UK website of GG doesn't even list DI, only the DLC, so …
Anyway, I can send you the coupon, but you'll have to wait until January to use it because it isn't valid yet. Also I'm not sure I got you on my Steam list.
~Sentinel Thoth~