your gp report will be valid for 6 months get that done early as that is causing all kinds of hold up, make sure your gp fills in the whole form and signs it, remember if your GP wont do it and you have to go through med cert or similar that all takes time and 4/5 months is not unheard of, I would be checking NOW to make sure your GP will complete the forms. I would submit application 3 months before expiry, your cert will automatically be extended by 8 weeks so long as it is received at least 2 months before expiry date, and if you do they must renew it in time. PM me for any detailed info. Fill in the form correctly, if is asks for medical stuff put everything down, if it askes for criminal history it means other than parking tickets every single thing since you were born!!!!! complete EVERY question if its not applicable put N/A or not applicable do not just leave it blank, make if paying by cheque make sure you have the correct payee, make sure the photo submitted is a new one and not one from years ago, if putting serial numbers on the form, make sure they are correct don't do what lots do and copy everything off old cert and forget that they sold 2!!! and they get included on the new application at which point somebody questions why you still have guns you said were sold.
Before anyone else states its not this or that, this is my job different force area but been doing this a long time unless your an FEO or work in FLU then NO you don't know