Scan through Craigslist & Mandy's. There are actor websites you can join, such as Actors Access, and many more. Sign up with an extras management agency, such as Casting Central, and spend all your time fighting other people for the opps. Or you can pay a small fee for a similar agency to do it on your behalf. Develop a relationship with a casting director. Figure out if you want to join the actors' guild, or stick to non-guild gigs. The employment will be sporadic, periodic, and not well paid. You will need another job, too.
That's pretty much the basics in Los Angeles. Your mileage might vary in Yorkshire.
I'm sure I could give a better answer if you get a little more specific with your question. It's pretty vague.