As of day one, Apple Pay will be accepted at 250,000 locations in the UK. That's the same number of acceptance locations at launch in the US, but the UK has fewer retailer locations in total which means that Apple Pay actually has higher penetration in the UK. Early merchants include BP, McDonald's, Starbucks and Transport for London.
Apple's biggest hurdle – transaction value limits – will be overcome soon. Currently, contactless terminals without the latest processing software can't accept Apple Pay transactions above £20. However, Visa Europe expects that 80% of the UK's contactless terminals will be upgraded to accept Apple Pay transactions of any amount by the end of the year. And presumably the remaining 20% of terminals would belong to smaller merchants that don't see heavy foot traffic.