HSBC was originally slated to be one of eight financial partners supporting Apple Pay in the UK at launch, but it looks like the bank pulled support at the last minute. The Apple Pay page now lists HSBC under "coming soon."
Replying to a customer's (our own Richard Devine) query, a HSBC UK customer service representative suggested that support for Apple Pay would be available sometime later this month:
@Ricker666 Hi Richard, we'll be launching Apple Pay later in July. ^SD— HSBC UK (@HSBC_UK) July 14, 2015
We'll share more details as and when we receive them.
Source: Twitter (HSBC UK)