Adam,
Just talk to your instructor about starting your PPL rather than continuing trial lessons! Everyone had to do it at some point so don't be worried. Then he/she will book you in some lessons at reception and then off you go from there! Easy!
And in regards to the medical, you won't need that till about 15 hours of training with your PPL and when you have completed your law exam. Though no harm in getting it before then as it will last you a while.
Raab