Many UK banks will issue IBANs (they are trivially calculated from the sort code and account number anyway), and payments can be made to IBANs - but for transfers within the UK, there is no need to provide an IBAN.

Note that it is still possible to check an account number without an IBAN: https://github.com/AntoineAugusti/moduluschecking

The issue here was not the account number being wrong, but rather the combination of sort code and account number being wrong - for which there is not necessarily a checksum digit covering the pair.