You might want to try Braeburn, nice crisp apples, and IIRC they're Scottish so they'd be quite hardy.
Plum wise, Victoria is a nice plum, and the trees are hardy, popular in Sweden so should take in an alpine climate, you can pick and eat them straight away, just keep an eye out for wasps, they do like them
Conference Pears are probably the most common in this country, although you might want to take a look at Pyrus Pyrifolia, a different taste to the traditional European pear, and grows at altitude.
Cherries, couldn't say, Morello (Prunus cerasus) is the most common in Southern England, not sure how it would take to altitude though, and it's a bit of a bird magnet
You might want to look at some Japanese breeds, it's quite hilly in Japan and they're very fond of their Sakura, although that's more orientated towards the flower than the fruit.
The clay soil is quite good though, anything that gets down to it will get a good anchorage, so less likelihood of going horizontal in the wind.
