DH and I fit the definition of squeezed middle class almost to a tee. We're both well-educated, both in salaried, professional, managerial positions, both earning the middle of the expected band for the jobs we do. We're also both the first in our families to make the 'jump'. He grew up on a dodgy estate; I was in a 2 bed flat with my brother and sister, courtesy of my mother's seriously screwed priorities.
We don't have huge amounts of spare cash. We can find, maybe, some years£500 a year for stuff like holidays. If we're incredibly careful, and don't need major car repairs to our 12 year old renault megane.
At the mo, that means a week in the UK, either travel lodge or cheapy cottage rental. It will never mean a week skiing for us all, or Nepal, or China. I (and DH) will probably never do those things.
What it might mean is exactly what people have been trying to point out. It might mean that DD does get to go. I wouldn't send her skiing, as I personally wouldn't see the value, but China? A chance to stand on the Great Wall, to experience that history and culture. Oh, yes. What parent wouldn't want that for their child?
School trips like that offer more children the chance to do things like that, not less. It's awful that not every child can afford it, but limiting trips to the very bottom of the financial ladder will hurt more than it helps.