Some medications have the side effect of causing photosensitivity which can lead to bad sunburn in situations when the sun is not particularly strong.
Sun cream on prescription seems reasonable to me in these situations.
Rather than laying down the law with blanket bans on over the counter items, relying on the common sense of GPs sounds a better bet to me. They should have a good idea as to their patients' financial ability to cough up.
I am sure this already happens as I have certainly been advised to buy paracetamol/calpol/sudafed etc in the past.
In any case, I am pretty sure that any savings made by not prescribing the likes of paracetamol and ibuprofen will be but a drop in the ocean. It is the kind of thing which grabs headlines and allows people to feel as though something is being done whilst there is little real effect.