It might be difficult for your child if they don't go on a class day trip which is part of the curriculum and will be the basis of the next few weeks work
If the trip is pat of the curriculum (like a geography field trip) and the school is a state school, then they are not allowed to charge parents.
If the school wants to take all pupils for something that will enhance learning, then they can ask for voluntary contributions in line with the law - it doesn't mean payment is optional, it means if you really can't afford it then you must not be harassed for it. They are also not allowed to exclude a child because the parents cannot/will not pay. So unless the school has a fund to make up a shortfall, or a body like the PTA stumps up, the trip may have to be cancelled.
If it's a non-essential add on like a ski trip, then it's fine to make it paying pupils only
Late communication is really shabby though - too few schools review their parent comms (at all, let alone from the point of view of the parent on the receiving end of it). One very good school near here did have a page on its website listing the major forthcoming trips, so even though costs unknown, you had an idea of what was in the offing.
But, OP, I bet they managed to ask the teachers who are accompanying the trip about their availability (esp if longer than typical day) with rather more notice than they are giving you to pay up. However, an undertaking to pay, even if the money comes in instalments, should suffice (unless there has been a very shoddy history of parents bilking)