My understanding is that it depends on whether the activity can be charged for, or whether a voluntary contribution is requested. There are fairly complicated criteria for deciding if there is a "charge" or a "contribution". For example, I think that a charge would be made for something like a GCSE field trip which is an essential part of an examinable course, in which case everybody is expected to pay, unless they are in receipt of certain benefits, in which case they would get a remission either from school funds or an external grant. If parents don't receive these benefits, then they have to pay and their children can't attend if they can't/won't. I don't know how strictly the last bit is applied but very importantly, with a charge you can only charge for the actual cost of the board and lodging/transport etc, not to susbidise children who can't pay. I think.
However, for other activities which aren't examinable, for example enrichment activities such as field trips, residentials, trips to the zoo etc, you can't make a charge and there is no compulsion or even expectation that parents will pay. However, you have to make it clear from the outset that the activity won't go ahead unless enough parents do pay. However, because schools are only asking for a voluntary contribution, you can pay however much or little as you like, and schools can request the payment for a greater variety of things, including costs associated with children not going or paying, eg supply costsfor those children who remain in school, or a subsidy for those children whose parents can't/won't pay. I think.
Here's a link to guidance and a model policy on Teachernet, which is a fabulous site for all those can they/can't they questions which often crop up.
www.teachernet.gov.uk/_doc/8181/ChargingforSchoolActivitiesbarnsleycasestudy.doc
I hope that helps. Obviously they guidance doesn't address the various rights and wrongs or the situation!