Is it possible to get someone from the CAA to look at your prescription and advise you before taking the full medical?
Yes, it is. Write to the Medical Department of the CAA, asking just those questions, and enclosing a copy of your optician's latest prescription.
Address is : Aviation House
Gatwick Airport, South Area
West Sussex
RH6 0YR
It might not be immediate, but you will certainly get a reply.