Sally ... can she easily be crossed with something that would produce a good beef calf? My mum has always loved Jerseys and I used to work with them a lot 
Loosey, yes Jerseys are renown for being able to produce good beef x calves. British Blue x from a Jersey is probably now the norm in dairy herds; it used to be Charollais.
I have been advised to break her in gently with something not too large for the first calf, either a Jersey (hoping for a heifer, unless you want a bullock for your own freezer) or an easy-calving beefy type such as an Angus. 2nd calf Angus, Limousin or similar (I am trying to get a Red Devon x, not sure if the AI has taken yet) and after that Charollais, British Blue - whatever you want.
I don't think you would have any trouble with her living out in Cornwall so long as there is shelter. (Well as long as you are not up on Bodmin Moor or something.) I know of Jerseys who live out all year in Somerset - and have milked same in December in a hand-built field shelter (actually built for goats) by the light of a hurricane lantern!