I recomend that you obtain the highest standard of medical you plan to need before starting any training. The main reason being as others have said it avoids you being in the situation of having paid a lot of money only to find that you will not meet the medical requirements.
Also since there are very few things that totally ground you these days, if you have a known problem you can adjust your training plans to suit.
Doing the NPPL declaration to save costs earlier on costs more later because as has been said you end up paying for two medicals. Unless you are getting on in years the Class 2 medical lasts for a long time.
If you plan to fly professionally and be paid for it then you will need a Class 1. Get the class 1 at the start. If your training is extended then simply use the Class 2 privileges after the first year until you need the class 1 bits again.
As for the air law. The CAA quite rightly requires that a student has adequate skill and knowledge before being sent solo. Unfortunately some people think that passing the PPL Air Law written exam meets that requirement which it most certainly does not.
One can score 100% in all of the Air Law exams from PPL to ATPL level and still not have the knowledge required to fly a circuit safely at a specific aerodrome.
Regards,
DFC