I have a couple of suggestions...
First off, and most importantly, your first backup body should be a buddy with good situational awareness. If you're anything like me once you get those "photographer eyes" going you're not aware of your surroundings in the traditional sense, and are more on the lookout for shots than danger. Also, two people are much less likely to be hassled than one.
Second. If you were in the US and in a state where constitutional rights are still intact (i.e., concealed carry is allowed), and your buddy it licensed to do so... then I'd say do so. But since you're in the UK, my only suggestion would be a sturdy tripod or monopod carried by your buddy. Normal people wont even notice it, but thugs certainly will; and will think twice about how badly they want your equipment.
Third. Hopefully the above two precautions would allow you to feel comfortable carrying your normal kit. But if not, you can still do great photography with a 400D & 50/1.8.
Remember, layers of defense. And if things do go pear shaped, your kit isn't worth your life.