Sorry to butt back in, I've read all the replies -
Cameron, Fred & WoofGang are correct, medical records (GP and Hospital) are classed as "excluded material" and fall under the protection of the Police and Criminal Evidence Act 1984 - Section 8 (1)(d)
http://www.legislation.gov.uk/ukpga/1984/60/section/8
The bit I don't fully understand, and I think Cameron may have been hinting at it too (correct me if I'm wrong, Cameron), even if they were accessed, which we've established they can't be, why are they not admissible or relevant. Your medical record is a list of consultations, medications, treatments, etc, why wouldn't this stuff be useful. For example; A women reports a rape to the police, she tells them that she has a sexually transmitted disease, wouldn't the fact that the suspect may have received treatment for the same STD be useful and relevant.