Having initially stated that Apple Pay was coming to the United Kingdom sometime in July, a leaked internal memo appears to give a confirmed date of July 14th for the service to go live across a variety of retailers.
An internal memo at supermarket Waitrose also indicates the same date that was suggested to Apple employees, lending confirmation to the date. Apple, as such, have begun enabling their points-of-sales systems across the UK and training staff in preparation for the launch date in a few days.
It is possible, however, that different retailers will have staggered launches and be spread across a few days, but it is widely expected that Apple Pay will go live on July 14th for most.
Apple Pay was announced at WWDC 2015 and this will be the first expansion of the service outside of the US. The launch will see a £20 cap per transaction to begin with, but will be updated in the fall to allow higher values. Apple is working to bring Apple Pay to Canada later this year, and China and South Korea in the future.