It's anyone with a HSBC or Barclaycard credit card, so probably quite a lot of people
People who don't care enough would normally have been OK paying with their Visa Debit which they will usually also have as an HSBC or Barclays retail customer...
Section 75 in particular has a lot of attention in the UK because over the last 4 years a lot of businesses have gone bust, and people who paid by bank transfer or debit card were shafted, receiving pennies on the pound (if that).