I can't specifically answer re: Levitra except our GP was reluctant to offer it initially and insisted on trying Sildenafil first, presumably because it is so much cheaper now it's patent has run out. I'm sure we are allowed 4 x 100mg of those a month.
As that initially helped we haven't pursued Levitra but a new surgery etc may well be looking to save themselves some money. I tried googling NICE guidelines for Levitra but it's all a bit vague and I expect you went down that route to check anyway.
Would the nurses on here know what the guidelines are I wonder. I'm sure Lyn knows.
Anyway, sorry can't help but perhaps I've bumped your request and somebody else may have the answer for you.