Savannah here from Shopify. :)
Great question! There aren't a ton of UK options for shirt fulfillment quite yet, but I've seen a fair amount of new apps popping up in the last year or so, which gives me hope that we'll have even more in the coming months. I'd recommend checking out:
Aside from Printful and Inkthreadable, those are essentially the only options when it comes to dropshipping from UK suppliers. That said, you can certainly set up a supplier that you find outside of Shopify as a custom fulfillment service, using this guide.
Let me know if you have any further questions!
Savannah | Social Care