I didn’t end up releasing this way in the end but I did do all the research and dealt with PPL before my fellow band members decided to go with Distrokid - which was rather disappointing but understandable.

Please forgive me if I over explain anything here but essentially you want to go to PPL’s website as they’re the ones who allocate ISRCs in the UK. It’s quite simple if you go to this link where they break down how it all works: https://www.ppluk.com/royalties/uk-royalties/recording-rightsholders/#isrc-registration

You can go here and register your IRSC code. There’s a few options but the simplest is “I want an ISRC but do not wish to register as a PPL member” it’ll then send you to a page with a big blue button saying “Start registration” : https://myppl.ppluk.com/s/qq

You might have already been through all of this and if so I apologise if it comes across as patronising but feel free to ask any questions and I’ll do my best to help