I really don't think you can expect the taxpayer to enhance the lives of poor students. They are already recipients of public funds on a large scale. It is unreasonable to expect the taxpayer to keep paying and paying and paying.
If a family has specific circumstances, such as disability, then the social services should be the budget that is dipped into, not the education budget. All the mechanisms are in place, but the actual amounts are subject to political control. It is woeful that a family who is unable to work because the parents are disabled themselves, or a fulltime carers, should be treated so badly by the benefits system.
The benefits system is a safety net. Able-bodied people don't starve but there is still the incentive to work. If you seriously can't work, then you should get more than the safety net. You should also be treated with dignity. It must be awful to go cap-in-hand and at the mercy of the headteacher.
I don't have all the answers but I do know that if your child is missing out on a PGL-type activity, there are many charities that fill this need, such as Falcon Camps.
If you could ask your DS which Biology specification he is doing, I would be happy to look up how he can fulfil the part of the spec that the fieldwork is supposed to cover. I cannot imagine that a fieldcourse is essential. We do Edexcel at our school and the that part of the Biology spec is either a visit to a facility (eg zoo, botanical gardens, university research establishment, farm) or the equivalent bookwork. There is certainly no need for a residential. If the school is only offering the residential, I think it would be reasonable for your DS to get individual tutoring on the book-based activity - you should press for that.
As for university taster courses, there are many free ones that are sponsored by industry. I remember looking up quite a few last year for DS, although by the time I did, it was too late to apply (famous last words for us).