What to do if someone else locks your bike up? Freeing locked bikes

But given that you are on Gower St, I'd jump a bus to Camden, visit the Leyland store there, and would put a battery powered angle grinder on credit card. Then I'd go back to Gower St, sit in a cafe with some visibility of the bike and charge the grinder. Before cutting the lock off.

The Leyland store on Goodge St. would be more convenient, but it's not huge so they may not stock one.