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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.

Most UK IBANs can be trivially derived from the sort code and account.

I have some accounts where the sort code and account are not directly embedded in the IBAN. So for those possibly there is a mapping table one could refer to.

You can _always_ derive it. For any case where you can't, something funky is going on (perhaps a common holding account IBAN where customer-specific information is encoded in the reference. I've seen this occasionally from TransferWise and Revolut)

No.

UK banking system is 6 digit sort code and like 8 digit acc.

The accounts are IBAN compatible though & it's derived from the above - but I can send money to a friend UK-UK without any IBAN stuff.

Maybe they don't check it.