The first thing to say is that it's a small industry. That's more than was expected. However, it needn't be all or nothing. Lots of people who work in the business do other things as well - sometimes mostly, sometimes occasionally. The most important thing is to be flexible - and have lots of stamina.
Pay can vary tremendously.
Members of the Extras' Union can expect a minimum of 65, plus overtime if there's an early start, a late finish, or a long way to travel. The best way to get some of this work is to join an agency. The good ones won't charge you a registration fee. Try to get to speak to other clients of the agency if they do. Not all production companies are the same, but here we don't really want to hear directly from people wanting work as Extras.