Days out with twelve of us could prove expensive. However, it doesn't have to be expensive. There is so much stuff out there that is free, and close to it. I am going to list a few ideas, including some stuff we do.
1) National trust membership- If you are a home educator, you can get this for just £36 a year. (though you cannot visit at weekends, and school holidays. Even if you are not a home educator, a family membership offers real value for money. Look out for a third off offers, which often include a free gift.
Belton House NT Last year:
2) Sometimes there are open days, when historical attractions are opened for free. These are listed on council websites.
3) Country parks- Often just a small parking charge.
4) Woodlands- Look on the woodlands trust website, to find places nearby. They can also offer activities such as den building.
5) Clubcard vouchers- If you are looking at a particular attraction, clubcard deals are very good value. Sometimes getting as much as 4x value for points. Otherwise look out for internet offers. We got into a safari park once, and had half of the children free, due to a similar offer that a friend told us about.
6) The beaches- Though some do charge quite a lot to park, there are often free places available, too.
I am sure there are many other ideas, and some I have forgotten. Please leave any extra suggestions in comments, and I hope in this time of recession it can be of some help.