Sending an invoice to your supplier isn't currently offered in QuickBooks Online for the UK version. I'll be sure to take note of this product feature opportunities for future enhancements. Once we receive some progress or enhancement for this feature, I'll make sure to inform you.
I'd encourage you to visit our QuickBooks Blog, so you'll be updated with our latest news and updates including product improvements. Just go to https://quickbooks.intuit.com/blog/.
Additionally, I'm including these articles for future use: