doobie- browse through the website "diggers and dreamers" (google cos i'm not sure of the exact URL). there are families who are living exactly as you describe.
Back in the 1970's I'd have had more sympathy with the pro-festival in term time argument. There were fewer events around then to attend at any time. However nowadays there is literally a festival EVERY week from May-October somewhere in the British Isles. Festivals have become BIG corporate business, especially since the growth of digital media over the last decade has meant many artists now rely on ticket income from performances, rather than recording sales.
I'm very pro-arts and "live" activities for children, and agree re the limitations of the NC, especially as I personally have a child who has great difficulty accessing the NC in it's standard format due to disabilities.
However sadly when you sign up for state school you also sign up to send em every day, unless a genuine family emergency or illness prevent you from doing so. No one has to use the state system, home education and private schooling are both perfectly viable options.
Take your kids to a festival - let them get muddy, learn weaving and circus skills, have a great time. Just do it in the state school holidays.
It is possible to have a delightful summer with the kids at a variety of festivals and achieve 100% school attendance. Try a festival other than Glastonbury - you'll be impressed at the sheer range of choice out there now. You may even meet me and bocjunior at a couple 