UK-issued BINs are getting harder to source since the Mastercard BIN Sponsor Plus crackdown earlier this year. Most of the major VCC providers (PST, Buvei etc.) primarily carry US BINs with some EU options. For specifically UK bank-issued cards in bulk, you're probably looking at either Wallester's white-label programme (they operate in the UK through their Estonian EMI licence) or Intergiro which issues Mastercard through a Swedish licence covering UK. Both require proper business verification though, so depends whether you need these for your own use or for reselling. What's the use case — ads, subscriptions, something else? That'll narrow down what BINs you actually need.