Apple has published a new FAQ support article, detailing how the new platform will work with UK merchants, as well as touching on the initial £20 limit.
All contactless transactions in the UK are restricted to £20 and Apple Pay will be no different. While the new payment system will work with any current contactless reader deployed by merchants, Apple Pay utilizes the same technology as contactless cards in the UK. No PIN is be required, but the £20 limit will be enforced on current terminals. This is at least the case until September.
Later in the year, the limit on contactless transactions will be increased to £30, which will include payments made through Apple Pay. While it's not a substantial increase (you'll not be picking up a Bentley anytime soon, Rene!) it will surely be a welcomed upgrade for those who frequently hit the limit for purchases. Unfortunately, the only way Apple Pay will bypass the limit in the UK is if retailers use readers that support Consumer Device Cardholder Verification Method.
CDCVM requires said new hardware, which will take merchants some time to roll out, but will eventually enable contactless payments to pass through referencing Touch ID verification. Until we reach that point in time, consumers using Apple Pay (and other contactless payment systems) will be restricted to £20 until September and then £30 thereafter.