+1 regarding the advice of Jan: get your medical first! Spending a lot of money and getting bitten by the flying bug only to find out that you cannot acquire a license for medical reasons would be bitter both financially and psychologically. Losing weight will very likely not be a problem for the medical and/or taking flying lessons, quite the opposite, but your AME (aeromedical examiner) will tell you everything about that.
Assuming that you pass the Class 2 or LAPL medical (these are probably sufficient for your objectives), depending on the weight of your flight instructor, it is possible that you have to train in a four-seat aircraft due to W&B (weight & balance) reasons (at least for some time), which is "merely" a financial problem. Up to about 15 stone you should be largely OK with a two-seater, but your future club/instructor will inform you. You can also play around with the two C150 on
Aviation Weight And Balance Calculators (N6518G and N48938) and some typical four-seat trainers (e.g., C172 or PA 28) for an idea of how your weight might relate to eligible aircraft types (cockpit space let aside, the 150 data also probably give you a reasonable estimate for other two-seat training airplanes like the C152, DV20, A211 etc.).
My advice would be: continue your losing weight and make an appointment with an AME to get your medical. Then take a trial lesson and check whether you like it. Then go for your NPPL
.
Blue skies
ACF