let's see . . 15 stone equals . . . 210 pounds.
In the US, that's like amateur size.
I am not in the least proud of being fat (200 lbs and 5'3'' female), but almost no one has seen my weight as too much for their horse to deal with. I , myself , will not ride a small horse. I don't need a rule to tell me that. But, most HORSE sized horses can carry my weight, and yours, without an issue.
there is a difference, quite a bit of difference actually, between how an experienced rider carries her/his weight and how a beginner does. So, that has to be considered. But, I think that you can perhaps find someone with a stockier horse, and work a lot at the walk to start with (which you would be , anyway).
best of luck. I think your plan of "mucking" , in the meantime, is a good one. A person can learn a ton just by being around horses and observing.