The UK doesn't have any banking secrecy laws — protection of account details falls under the Data Protection Act 2018 but the general principle was established in common law by Tournier v National Provincial and Union Bank of England [1924] KB 461
Okay, I was mistaken there, that seems to be country-specific; and according to http://www.legislation.gov.uk/uksi/2017/752/regulation/90/ma... they should have disclosed the contact info (or sufficient information to sue) at the payer's request. That would have prevented the need for the first (but not the second) lawsuit.