I used to do it while at college as I was too lazy to get a regular part time job at the time it paid around £65 per day up to maximum 12 hours, then there was an overtime rate but I was never kept for longer than that.
It's boring and unpredictable, I was hoping to have a career behind the scenes in TV production which is why I did it. I may get 3 jobs one week then nothing for 6 months. I found a reputable local modelling and acting agency, contacted them and asked if they dealt with extras - they didn't but could put me in touch with somewhere. Remember never to pay for representation if you aren't sure about the calibre of the agency
My daughter recently did some extras work for a really big TV production. I saw the ad on the Facebook page of a local extras casting company (I'm not in mainland UK so details are probably not of any use to you) and sent in the requested details.
I would say that it appears to be very boring a lot of the time and is not well paid (DD earned about £50 per day and I was paid as well for being her chaperone as she is a minor). The adult extras spent a lot of time hanging about waiting for their call and ever changing filming schedules meant that sometimes they were in uncomfortable costumes/cold/wet for long periods of time. They also couldn't change their appearance for the six month duration of filming...no haircuts/colouring hair etc. Because DD was a minor and there are very strict rules around children working, we got better treatment...dedicated staff to look after us, our own trailer on set etc. But it's anything but glamorous. On DD's first shoot, I spent hours in the rain, ankle deep in mud in the middle of a forest. Another day, we were halfway up a mountain in the arsehole of nowhere. It was an experience...but not sure if I'd want to do it regularly.
I used to have a high paid stressful job, dh and I decided Id give up and we'd try ivf again (we'd done it before whilst I was working and it failed) anyway, I've not been working for 4 months now and we are both undecided about doing ivf again, so I'm basically a sahw! Anyway, I've toyed with a few career change ideas but nothing has really grabbed me yet, but I'd like to do something... I'd always loved acting as a teenager and thought it might be fun to do some work as an extra. But I've no idea how or where to start, there are a few websites, but I don't know if these are dodgy or not :S Anyway, anyone done/doing this? Any tips?