In 2011, I rode my qualifiers (Shaftesbury Spring 200k, Elenith 300k, Brevet Cymru, Bryan Chapman) on 67" and rode PBP on 69".
I can spin down hills pretty well so didn't feel too under-geared. On the Brevet Cymru at the weekend, I was comfortably able to maintain 50-55kph downhill, and got up everything, on 65". Many of the riders who recommend bigger gears don't seem to be as good at spinning. However, as I've put my name down for the Vedettes this time, I'll probably gear up to 74", with the option of turning the wheel round to 69" if it looks like a tough headwind is in store.
FWIW, I rode LEL 2013 on a mixure of 69" and 65". Either would have been fine for the whole ride, though probably 69" would have been a better overall choice with the long flat stages.