You will not necessarily “fail” your class 1. There might be things that need investigation for example. If things are outside “limits” then you know what you have to get to and see if you can do it.
The Major failing point is eyesight and colour vision. Not a lot you can do about those, but you could download the CAA extended eye examination data sheets before hand and see an optician with them – give you a pretty good idea.
In reality – don’t panic, most averagely fit person will be able to get a class 1. Enjoy the day – odds are you will meet some great like minded people at Aviation House that are just as nervous as you – and will come away with a very un inspirational piece of paper with Class 1 written on it the same as you!!