First and foremost, horses are herd animals so need company of their own kind to be happy. If you don't want two, perhaps you can find another similar family locally to share grazing?
Secondly, I have Fells and can't rate them highly enough. My Fells are hardy, clever, sensible
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I guess Exmoors would be equally suitable if you're southerly rather than northerly; or Highlands if you're in Scotland. (If you're in Wales, you'll have to ask someone else - there are just too many types and categories of Welsh ponies for me to understand
.)
All native ponies are "good do-ers", meaning you have to be careful to not let them get too much rich grass and mustn't feed them much if anything in the way of concentrates.
Another approach, which I would certainly explore, would be to see if your local riding school or trekking centre has a pony ready to move on to lighter duties. Others more experienced with older ponies will be along shortly to advise you on vet checks, likely health issues, etc, if you went that route.