I know this isn't going to be for everyone but living in London usually a day out anywhere can cost a fortune.
You apply for the tickets you want and find out 48 hours (or sooner) before if you have got them. I have seen Britians Got Talent and Saturday Night Takeaway and both times we had a really good laugh and all for free.
You can also apply to be on the shows but I have never done that so can't shed much light on it unfortunately.
Hopefully helps someone :)
From the website:
We are the only official and authorised ticket unit to handle our shows. Our free tickets are not available on any other website or through any other company.
Applause Store is the award winning, one-stop-shop for free, official television and radio audience tickets for thousands of the very best music, comedy, chat, sitcom and entertainment shows produced at many different studios and locations all around the world.
Depending on Broadcasters schedules, we officially and exclusively represent television's hottest shows produced for most of the major channels including BBC One, BBC Two, BBC Three, BBC Four, ITV1hd, ITV2hd, Channel 4, E4, Channel 5, Sky1, MTV and all Satellite & Cable channels. Plus, at certain times throughout the year, we distribute free tickets for pilots, specials, award shows and other one-time-only television events!