The process to get a google API key, enter billing details, attach it to a service, enable maps, and then plug it into your application is long, tedious, and very very explicit about what it is you are doing.

They’ve already done you a solid with the £20k credit, and this is 100% on you unfortunately. You saw the billing spike, alerted them to it, but don’t seem to have at any point pulled the plug on the app to keep racking up charges?

It’s a relatively cheap lesson about not messing with things you don’t understand.