Probably not what you want to hear but be prepared to pay far more than the basic initial class 1. I'll break down my expenses for an example:
Dual UK/EASA initial class 1 - £1007
UK CAA deferral (fee that you pay if you are not assessed as fit on the day of the initial) - £105
Psyhciatric review 45min - £450
Opthamology review - £150 (originally £300 but the appointment took 15min so the Dr was kind enough to reduce the price) + £112 for pachymetry (10min)
Cardiology review with 24hr holter ECG - £460 30min
I had a bit of a complicated childhood with lots of trips to hospital appointments to be fair, but I wouldn't be surprised if more people get deferred to specialists than those that do not. Don't be disheartened if you get deferred as it is not over, but make sure you can afford it. Telling the AME that you have/had X issue but no/few symptoms is not enough for them - they (and the CAA) will probably want to see the documentation from your doc to approve this claim.
Besides that, just read the instructions emailed to you carefully i.e. no contact lenses 48hr prior to the appointment, create your Cellma account and pay the CAA fee within the suggested timeframe, drink fluids for the urine sample etc etc etc.
Best of luck.