Alright – yeah, you’re right – with co.uk domains you’re right, you need to name the receiving registrar using the IPS TAG.
This thread https://community.shopify.com/c/Technical-Q-A/IPS-TAG/m-p/902716/highlight/true#M35754 points out, that it might be MARKMONITOR. Can someone from Shopify confirm this? @Nick maybe?
Hope this helps,
Mario