My bank (BOFIIE2D) now gives me the option of instant or standard payments. Both cost the same. And in both cases the money leaves my account instantly. I can just choose whether it arrives at the recipient instantly or after a few days. I have no idea why standard is still an option. Why would I want that?
Most likely answer is just that BOE doesn’t have a strong product design culture.
But there is another reason why some may prefer standard: due to the instant payment rules banks are not able to hold inbound payments for screening so if a payment trips a flip that requires screening then it will be rejected. Other payment rails will typically result in the payment being held for a manual review where it will be released shortly later. Instant payments will be rejected if not crediting the recipient in a few seconds.
As this disproportionately affects people with names in common with sanctions lists, to send money to/from those people sometimes a standard transfer is the only option.
That makes sense, then. It is actually a useful feature (especially when you're not sure on the spelling of someone's name, etc.).