I think in London, the rate is close to 25+pounds per hour and can increase depending on experience.
That'd put web developers in London on approx £50k. That might have been true during the dot.com bubble, but these days high twenties is more normal.
I think that really depends upon what level you're working at. An entry-level web developer (PHP/HTML) might earn something like £15-18K in the UK. Some companies will pay more though. There's more than one side to the industry too.... you can be a designer, you can write (X)HTML, or you can be a programmer.
If you excel in a particular field or you soley handle a part of the project then you should expect more. Once you gain experience in the field and really impress people I'd expect to look for £30K+.
Granted, you can earn a lot more doing it freelance but the work fluctuates and the market is fairly saturated so I wouldn't call it "stable".
The dot com boom was definitely a time to make big-bucks if you new what you were doing... it's not so easy now.