From my tiny bit of knowledge of these things, I think the idea behind it is to prevent someone having an industrial accident and trying to claim for a previous unrelated injury. For example, you may have had years of back pain which the company has no knowledge of, then have a fall at work and sue them for thousands for a back injury sustained on their premises. Does that make sense?
That said, it sounds a particularly intrusive form, and you certainly shouldn't have to give weight, GP's details etc. Presumably the company has an emergency number and if you collapsed at work they wouldn't be ringing your GP anyway.
There should also be some sort of assurance that the form is confidential. As D74 and Coops said, it could be related to some sort of income protection scheme, but that should be made abundantly clear to you and you should be issued with a private/confidential envelope in which to return it.
Depending on the size/structure of the company, I would ask in writing what the purpose of the form is and then go the ACAS route.