Bacs is the UK's batch direct-debit and credit rail. The settlement cycle is "Bacs 3-day": instructions submitted on day-1 are settled on day-3, with the day-2 "delay" used for the recall-and-return window. The format is a flat-file text schema (Standard 18 / Standard 67) — *not* ISO 20022.
UK direct debits run on Bacs; UK real-time push payments run on FPS. UK businesses operating against UK customers typically use one or the other, with Bacs the right pick for high-volume recurring collections (utility billing, subscriptions) and FPS for one-off real-time push payments.
The ISO 20022 migration of Bacs is on the New Payments Architecture roadmap but lags FPS by years. Today, integrating Bacs is an integration against the Standard 18 / 67 text format, not against pain.008.
iso-compliant does not generate Bacs Standard 18 / 67 files today; UK ISO 20022 coverage is the Phase-3 scope.