Apple Pay will reportedly go live in the United Kingdom on Tuesday, July 14. Apple has apparently informed retail staff in the UK to expect the system to go live on that date and supermarket Waitrose is reportedly distributing internal material with a July 14 date for Apple Pay as well. From 9to5Mac:
Apple will also begin training its U.K staff on supporting Apple Pay on July 12th. Apple has begun enabling its mobile point of sale systems in England-based stores to take NFC payments and has been preparing materials to promote the Apple Pay launch in stores. It is plausible that different stores will have different launch dates, but the presence of the same, at least tentative, date makes July 14th a very possible widespread start date.
Apple announced Apple Pay availability in the UK at last month's WWDC conference. Additionally, as with all contactless payment systems in the country, Apple Pay will be limited to transactions of up to £20, though this limit could rise in the future.