Speaking as someone who is, um, well over the weight limit any riding school would take ...

I think, seriously, that if riding schools don't get their act together over this, eventually they will simply go out of business. Whether they like it or not, the population is getting heavier. Opinions do seem to vary enormously - some private individuals without any apparent axe to grind, and with great expertise in horses, claim that native ponies could carry the weight the OP is quoting and that a heavyweight ought to be able to carry a 25 stone man for a day's hunting. And then there are the riding schools who won't take anyone over 14 stone.

I'd say there is a niche out there for riding schools, trekking centres etc who work on the basis that riders can be heavy and simply buy or (for someone down the line eventually perhaps) breed horses that can carry such people. To say that heavy horses, or perhaps heavy horses crossed with cobs or cobs crossed with mediterranean donkeys, can't take that sort of weight, is surely plain silly. How much did an overfed medieval nobleman in full armour weigh? How much do two people weigh when filmed on a galloping horse in a historical blockbuster? How much weight do the poor donkeys around the mediterranean carry, and for how long, with far inferior food and care than your average riding school horse? Surely the issue is how much carrying of that sort of weight the horse has to do? So, buy the horses, rest them for longer and charge a higher rate for the lessons. There has to be a niche out there for someone, and sooner or later market forces always prevail.

Liz