I remember reading somewhere that animal charities abroad take out of date medicines - today I've found some Metacam (use by 2008).  Would this still be effective?

Once Metacam is opened, it has a very limited shelf life.  (only a couple of months I think)

> I remember reading somewhere that animal charities abroad take out of date medicines

You could ask your vet. Ours have sent previously dispensed medicines to Pakistan for an animal charity appeal. I don't think the meds we returned were out of date though, just opened and not finished after the death of our dog. It might be worth checking, then at least you'd know what their policy is.