For my kids, I don't think it would be a problem to miss a week or two of school but it isn't just about my kids, is it? There are 20-30 other kids in the class too and if the teacher has to spend extra time with my child to help them catch up, that's less time with the kids whose parents haven't broken the rules - hardly fair. It's also more work for the teacher and many teachers are swamped as it is. If the trip was something really special - perhaps a foreign trip to somewhere with a rich culture (and where you won't be sitting on the beach all day or eating in restaurants that serve English food!), perhaps it could be of enough value to the child that the affect on the rest of the class could be forgiven, but I think probably most of us don't take our kids on holidays like that! You'd probably need quite a lot of money to do it so you can probably afford to go in the school holidays anyway. I'm not sure a trip to Butlins or wherever is of any real value you couldn't add by doing some day trips at the weekend or in school holidays.
I don't think it's right that companies can charge so much more during school holidays and I'm all for that being sorted out, but until it is we will either stump up the extra and go in the holidays or not go at all.