The first skin is to register British accounts, while the second one (global) accepts players from all over the world. The “dot-UK” skin is regulated by UKGC, and the “dot-com” version has the Isle of Man license. They, though, share the same player pool.
In terms of poker games and promotions, they are practically identical. The important difference is that the United Kingdom Gambling Commission has set some specific regulations, for example, a monthly loss limit. P2P transfers are also not available for British accounts.