Cessna 152's are the standard a/c to learn in at most flying schools, so no worries there.
Wrt the medical, as has been said, get that out of the way asap - you don't want the (unlikely) situation of having had 10 hours or so of lessons and therefore spent nearly £2k only to find out you will never get your PPL due to medical reasons, however...
I dont think anyone has mentioned the NPPL as yet, which is a reduced version of the PPL in that it is limited to the UK and so on, but what it does mean is that you dont need the class 2 medical for it, only a GP sign-off to say you are fit to drive (and thereby fly). So, if you did happen to fail your class 2 medical, you still have the option of the NPPL, which may or may not be suitable for you.