Which part of the canal was it? There was a spate of bike-jackings last year in NE London. Make sure the London Cycling campaign know about it. There was a fairly successful "bike patrol" by people who got pissed off waiting to be jacked. They scared of the few youths who were responsible for the trouble.
If you have a metal U-lock, they make excellent defensive weapons. I keep mine attatched to the top of the rack with a bunji chord. Its quick to get at, and totally legal.
In the UK, the laws about carrying things which are primarily weapons are very strict. The defensive characteristics of a device can only be secondary to its normal function and you must have a clear reason to be carrying whatever it is, hence metal U-locks, Maglite flashlights, but not pepper spray or Mace. If you use a primary weapon for self defense yopu will almost certainly go to jail, whilst your attacker will skip his community service punishment. I can alreader hear Hunter going for his gun....
In my small English town there have been a couple of handgun-muggings on my bike route, which is really scary.