I was under the impression that you had to have a valid medical to fly solo (but a very rapid trawl through my book hasn't turned anything up). I took mine before training, because I suffer from a couple of conditions that I feared might be an issue (migraine & asthma). I didn't want to shell out for tens of lessons only to be told I couldn't fly!
My school told me that it is no longer required to have your air law to solo, but it is strongly recommended by the authorities. Therefore, they want me to do it first (not that I'm there yet!).
HTH