I was able to successfully buy a ticket using Uber Money and Uber Credits this morning.
Similar to James H’s experience above (post 379), just before clicking Buy I went into payment options and manually re-enabled using Uber Balances (which keeps switching itself off).
Back on the Buy screen, the interface then implied I would be using £0.00 Uber Balances plus credit card, but in fact the whole fare was paid from Uber Balances (a combination of Uber Money and Uber Credit).
Hopefully this is a bug which will be fixed soon.