Maymom has them, I don’t know if it’s available in the UK. I got it off Amazon works with standard and SlimFit

Maymom Valve Compatible with... https://www.amazon.com/dp/B07MJ8ZNB7?ref=ppx_pop_mob_ap_share

Also PSA: try wearing a waist trainer belt over your chest while using wearable cups-I found it helped increase my output, similar to Pump Strap

You could probably also email them. I found them to be fairly responsive.