An IPS tag is a label that applies to any Registrar that registers .UK domains.
If you want to transfer a .UK domain to one Registrar to another you need to know the IPS tag of the new Registrar and modify the IPS tag, before placing the request.
I did some research and Shopify doesn't seem to be an accredited Registrar for UK domains. In fact they used MarkMonitor registrar themselves to register their domain shopify.co.uk
Could be that they partnered with MarkMonitor to manage UK domains for Shopify too, and in this case the IPS Tag would be MARKMONITOR. But this is just a supposition, and need to be confirmed by Shopify
Anyway, another possibility is to keep the domain to the same Registrar you are currently using and just update the DNS record of the domain to point to Shopify.
You can find more details at this link Connecting a third-party domain to Shopify