As far as I know, Stripe SEPA is only available in EUR currency[0], so this nearly excludes the UK completely. At least for us, all SEPA payments were outside the UK. Probably 95% from Germany and Austria, and the rest from a few other EU countries. Not a huge sample size, but we've been running with SEPA for a few months. We did an A/B test for a a few weeks as well, and basically observed no hit in total revenue when we offered SEPA or without it (only cards and PayPal), so that was the main reason we pulled out the plug. Not just the chargebacks.
Another thing I should mention is the async nature of the whole thing. Unlike cards that get authorized/declined instantly, we're talking days or weeks with SEPA. For a monthly subscription, this eats into the first month easily.
Again, we're talking about private individuals here (B2C), typically students in our cases, and as I mentioned predominately from Germany, so YMMV a lot with other customer types or countries.
[0] https://stripe.com/docs/sources/sepa-debit#create-source