Those dozen banks cover about 99.99% of people who have a UK bank account, so in practice, sending money to other people in the UK is free and instant.
Pix isn't really comparable to SEPA Instant or Faster Payments, and the adoption rate shows it.
is a very generous word to describe banking websites and apps!
Sending a payment to a new person with Santander is going to take at least 5 minutes and 3 round-trips through SMS 2FA.
Instant seems a good description.