When I knew I was going over through BUNAC, I just searched online for Edinburgh Fringe Festival jobs and found a message board post from a production company looking for PA's. I emailed them, had an interview when I arrived in London, and a few weeks later I was running shows in Scotland. Although that was back in 1999, I'm sure you can find something similar now. It's like all networking: just start talking to producers to see where they need help. Because I got that position, the people I met and worked with picked me up for other things once we all returned to London. There's also a weekly theatre industry newspaper (The Stage, I believe), like Back Stage in NY, that was a great resource. I absolutely loved it over there, and having the freedom to earn money, make your own schedule, and not worry about going to any classes is fantastic.