You can be rejected for missing your medical appointment as each costs in the region of £100 & everything after the medical costs more than that. All you had to do was ring up your Careers Adviser to ask beforehand, so I'm afraid sympathy is in short supply.
As it is, you've now deprived someone else of that medical appointment.
Had you arrived for your medical, having not had your eyes tested, they would have simply given you the medical examination & waited until your eye test forms were received before passing you fit.
Start apologising, it may help. Bear in mind if you are permitted to proceed, then any other evidence of unreliability will be the end of your application.
It's called "Personal Admin", indications are you'll fall foul of it in training if you get that far.