Hi Joe, great suggestion and question! Totally understand what a pain this can be for small business owners. Currently, you can upload an image of a receipt against any transaction within Monzo. 
You can also categorise the transaction against specific categories (we’re exploring new categories and you can take a look here), and give some feedback!
We’re also exploring how we can automatically export those receipt images into accounting tools (like Xero and FreeAgent), and it’s something we’d definitely like to do in the near future.
Receipt OCR is a little further out, but hoping that with the above it’s a step toward making things easier for you!