I work for an agency
What I will say is (and I made this error before once) NEVER EVER pay any money upfront to them, not even for photoshoots. They may try and fob you off by saying "We need to take ?200 for insurance". BULLSHIT! Chances are, you won't get anything from them and they are just out to scam you!
Unfortunately, I am in Scotland so not too sure about the ones in England lol. But what I will also say is, a good agency will work with you. They won't pester you to work days you don't want, you can work when you like etc. Never be forced to do anything you don't want to do. And a proper agency won't take all your money. One I rung before I found the one I'm working for now, was run by a foreign girl. Now I have nothing against them, but I simply couldn't understand her at all, and had to ask her to repeat several times. You were expected to do your own pics or pay them ?160 for a shoot (my agency's was free), and they took 3/4s of your earnings. Any like that, run. My agency only take a fraction, I get 3/4s!