I agree there doesn't seem to be rhyme nor reason as to how you are supposed to renew. After years of doing it on paper I was delighted to be able to do it online, and to be able to use my passport photo. But last year it was back to paper - I looked for ages to find a way to renew online (I have no other relevant medical conditions) and to use my passport photo (less than 12 months old at the time) but to no avail. The only change to my circumstances I could think of was that I'd turned 70 since the previous renewal.
All forms completed and new photo taken by my OH (fortunately we have a lot of white walls in our house so not too difficult to meet the required standard, and the photo was much nicer than my passport one!), the package was despatched and the new licence appeared within a fortnight. Like helli, I got the letter to say it was on its way after the event.