There's no excuse for anyone in the UK to be underhorsed as they've got such a huge diversity of horses and ponies to suit any weight and height of rider.
I'm not the one arguing that 10% is a maximum weight, or 15%. Take Mia's new owner (since 2015). He's got to be 220-230 in his socks. When he isn't riding her bareback, he's using a roping saddle. 35+ lbs. Mia's maximum acceptable rider weight, including tack, as being pushed by the folks I quoted earlier, is 135-153 lbs (15-17%). He'd need a 1500-1600 lb horse. But 900 lb Mia has cheerfully raced 4 miles carrying him against racing horses just because she didn't want to quit!
Other than color, those three horses are identical to their half-brother Trooper: 3/4 Arabian, 1/4 Appy, about 850 lbs. One rider is light, maybe 135 lbs. The other two outweigh me, so around 200 lbs? The sons don't want to breed their own horses any more, so they've been buying locally available Quarter Horses instead. I asked last week how it is working. The Dad says the big horses just aren't getting the job done. They tire too quickly, don't rack up the miles, need more days off, eat more and need to be pulled back to the home ranch for rest more often.
Bandit WAS ridden too hard. He weighed 800 lbs when he arrived (vet's estimate) and he'd carried over 300 lbs on training runs. He braced his back like an I-beam when he arrived and I believe his knees have been abused. There ARE limits! But the Arabians I've met don't blink an eye at carrying 200 lbs.
Here is what I believe is an easy test: If your horse can carry you with a flowing, supple back, in a relaxed trot or canter, you aren't too heavy. If your horse stiffens when you get on, you have a problem. Get off!