Not sure what the UK is like, but I passed a class 2 medical here in Aus. That is the one with the poking and prodding etc. I am not quite at your weight, but not far off it. 6ft and pushing 110kg here. Admittedly, I was not this heavy when I started training, but the reason I took up flying was because I broke my knee pretty badly in a trail bike accident, and have since packed on a lot of weight because I cant do much exercise.
Anyway, you will be fine in a C172, and I have done a 3 up trip with me in the right seat of a PA28 and weight was not an issue. 2 up for training wont be a problem. Likewise, for a non commercial medical I doubt you will have an issue provided you dont have any other major medical problems, but then again it may be different in the UK.