Hope you enjoyed the rabbithole!
Actually it's very common for choral scholars to study subjects other than music, and not uncommon for organ scholars as far as I know. Some colleges are busier than others; and some subjects with busier workloads / lab times likely to conflict with practices, are not able to be combined with choirs with bigger commitments. But many of the choirs have 3, 2, or even 1 service a week plus a practice or two so to an extent students can match themselves to the commitment they are interested in. And there are lots of opportunities to join some of the lower commitment choirs after starting university.
DD has sung in choirs regularly since she was 7 so it's a part of her life. She's applied to one of the 3 services a week colleges that does significant recordings and touring out of term as well. She feels that the commitment is likely to help keep her disciplined in how she uses her time. (She also rows and we're looking forward to seeing just how much she manages to keep up, depending on which university she ends up at next year!)
DS hasn't chosen his A levels yet though I think maths / FM are a given. Music at university is possible but so are maths and computer science and probably a few other options!