No lock? A good lock might have bought you some time, especially a high quality lock securing your bicycle to a fixed object, even a low cost u-lock can't be broken or cut quickly without making a lot of noise.

That said, even the toughest locks and chains only take a few minutes to cut with a battery powered angle grinder, if it was my most valuable possession, it wouldn't leave my sight if I felt it was unsafe.

Bicycles are great, and usually replaceable, always factor that into your travel expenses.

Sorry for your loss, thieves are everywhere.

London is world famous for many reasons, especially theft.