I think they also likely used them to plug into certain payment schemes initially. Probably CHAPS and international payments.
The sortcode they use did come from the NatWest agency banking range, rather than a totally unused range, so they must have been processing at least some payments (probably beyond just cash deposits) on their behalf).
Indeed, you are right about that. However, I think it’s not always the case now? The 08 range, for example, is nominally all the Co-op Bank, yet Virgin Money now seem to clear their 08-60-64 sort code themselves as it is listed separately and comes up in the Faster Payments Checker as owned by Clydesdale Bank PLC.
Yes, and this sort code comes up as enabled for Cheque & Credit Clearing - Starling, of course, don’t issue chequebooks but do clear cheques now they’ve joined the Image Clearing System and enabled cheque imaging in their app.
The Wikipedia page mentions it is reversed for “utility banks” that don’t use cheques and links to this document from Pay UK - unfortunately, although the phrasing “Utility Bank” does appear, there is no mention of cheques in the document so I’m not sure where the idea originated. However, I don’t think any bank with a sort code allocated in the 04 range actually issues cheques - Starling participated in the scheme, but still doesn’t issue cheques, so perhaps the limitation just applies to not issuing them?
However, I agree with you that it doesn’t seem like there would ever be a technical reason to prevent certain codes from using certain payment schemes - so presumably cheque clearing could always be switched on for an 04 sort code even if the range was originally intended for use with banks that don’t issue cheques.
My guess is they just needed to use a spare code for fintechs (given that there are and were going to be a lot of them) so they could hand out new sort codes not connected to traditional clearing banks. Given that 04 was unused, this was an ideal number to bring into use. Since no fintechs ever joined the paper cheque clearing process, and initially none of them cleared cheques themselves, it was also easily inferred that any 04 sort code didn’t support cheques. The code may then have been blocked in the clearing system to avoid fraudulent cheques being claimed against non-cheque clearing banks, but I doubt it?