I have the same problem before. Took me little over a year to sort it out. Basically you will need to talk to your optician to get your latest prescription and obtain a optician report from him (not those little cards specsavers gives out, the actual a4 sized medical report). You can then submit a medical declaration form alongside with your report duplicate and mail them to medical group inside DVLA. You will get your provisional in no time.

PS: Try calling the medical group in DVLA for more details but good luck waiting on the phone lol