Having owned scooters and motorbikes in London for 15 years now (6 in HOP, 5 in Camberwell and 4 in Notting Hill), the only time I've had a bike stolen was when it was alarmed and immobilsed, with a chain round it and a cover on it!

It was, however, not chained to anything. It was found 2 days later dumped by the side of the road as it couldn't be started.

Since then I've always used a ground anchor to chain my bike to, inside my front garden. I've not bothered with an alarm since, people ignore them and they drain the battery.

Good luck riding in London and be safe.
KK