I really hope Monzo is looking at partnering with Post Office rather than PayPoint/Payzone.
I used to make a lot of cash deposits to top up prepaid cards etc and the PayPoint experience is not very Monzo at all. Most of their many locations are tiny corner shops where the proprietor is earning 50p a transaction to process someone’s bill payment for maybe £20-£50 - tap on Bill Payment, scan the barcode and enter the amount the customer is paying in cash. That’s 50p for 30 seconds’ work and the customer probably bought other stuff as well.
When i rocked up with my unknown brand of prepaid card and £500 in cash to put on it 90% of them didn’t have a clue what to do with it, let alone want loads of cash in their till. Mostly there was a 5-10 minute faff around on their screen, stopping and starting for whenever a real customer came in and wanted to pay, ending up in the manager being called and spending more time awkwardly figuring out how to do it. Some succeeded, others just said “sorry it doesn’t seem to work” 
Now the Post Office… whilst everyone knows you can get awful queues and they can be fewer and further between, their staff are a jack of all trades carrying out dozens of different types of transaction and they are used to dealing with cash over £100.