Preview at WWDC likely in June, followed by September launch.
Barclays Confirms it Will Launch Apple Pay by Late March
The launch date was provided by Barclays CEO of Personal and Corporate Banking Ashok Vaswani in what appears to be a legitimate email sent to customer Oli Foster-Burnell, and later reported by Engadget. We can verify the executive has emailed customers about Apple Pay in the past.
Barclays is the only financial institution among the "Big Four" banks in the U.K. that has yet to adopt Apple Pay. The country's participating issuers include the Bank of Scotland, First Direct, Halifax, HSBC, Lloyds Bank, M&S Bank, MBNA, Nationwide, NatWest, Royal Bank of Scotland, Santander, Tesco Bank, TSB and Ulster Bank.