PokerStars holds its license with the UK Gambling Commission (UKGC). The Gambling Commission was set up under the Gambling Act 2005 to regulate gambling in Great Britain.
You simply need to log in to your existing PokerStars account, then follow the prompts to upgrade to the PokerStars UK version of the software. Your username and password don't change and you will still have access to all your usual games and promotions.