I have been an extra on a couple of things, but I got the work through knowing the director or in the case of a Channel 4 drama, the Art Director (I was dressing sets in the morning and an extra in the afternoon). I got paid more for the set dressing than for being an extra. I was in a commercial last year, but I was working on the commercial anyway, so didn't get paid for the extra work.
Don't pay to be put on a register, there is no need to pay.
The Stage magazine may be of help...
http://www.thestage.co.uk/