Have my contract employment been breached (United Kingdom)?

Yes they can do that, your salary rise is following a completion of the 12 month plan which means you have now completed the 12 months so they can take as long as they want to decide if or if not you will get a raise, it does not say you definitely will get a raise after 12 months it says 'following successful' completion, they might decide you didn't do enough progress to deserve a rise. Good Luck.