It's mine, I tell you. My own. My precious.
Do I have the keys? No. Can we pick the lock? Also no.

I contacted a company who stocks spares for UK payphones, asking them to send me a set of keys to try.

They replied with "That will not be possible" and the following photo.

@jonty Try this guy!
@geospacedman @jonty Damn, beat me to it!