I haven't tested this yet but many Ancestry records can be accessed for free using www.ancestrylibrary.com in participating public libraries. Your local authority may or may not subscribe and some libraries in its area may not be included in the subscription. Otherwise, if you plan ahead, you can take a 14 day trial to use from home but be sure to cancel before the 14 days is up so that you don't get billed. I believe it is keyed on credit card number.
Some of the records in the National Archives contain details of height and chest size for instance and also correspondence in which the Army is stonewalling against paying out for disability etc. (if you thought that was new!). Don't know yet whether all that stuff, which is viewable on microfiche at Kew, is now online. Medal records ARE online (and have been for some time) which can at least give the most basic detail - but some people maybe did not claim, perhaps unsurprisingly just wanted to hang their boots up and forget all about it.
If you find a man has been transferred from infantry (say) to Labour Corps this may give a clue to a medical downgrade.