I got mine via mate’s presale as I’m a big fan and didn’t want to take the chance.
I don’t think it’ll sell out and definitely not quickly. I saw The Julie Ruin at The Electric Ballroom and The Koko relatively recently and Le Tigre many years ago. If you’re a fan you’ll see her no matter which band she’s in. I can’t see many people being put off that it was The Julie Ruin/Le Tigre and similarly can’t see a huge bunch of people who’d only see her if she was in Bikini Kill.
Having said that, that looks like their only UK date and not a UK tour. Many people may come over to that there London for it.