Its interesting that I picked-up this old thread this week, because today on the BBC news they were saying that some recent research carried out by Imperial College London has indicated that 10 portions of fruit or veg per day can significantly improve resistance to heart disease and some cancers, and this obviously has potential to improve life expectancy.
Regarding your enquiry about home-grown produce, Kathryn, our supermarkets do carry quite a lot of local stuff in season, but the UK climate is not altogether conducive to fruit growing, so of necessity we have to import much of our requirement from abroad. Also, because we have a large population living on a small island, we have always had to import large amounts of food just to feed the population. For example, in our shops we have large amounts of butter and lamb from NZ.
At this time of year (it is still winter here) a lot of our fruit comes from more southerly climes. My particular favourite is strawberries: in season we can buy our own home-grown examples, but once the season is over, we start to import them first from Holland, then Spain then finally North Africa - usually Morocco or Egypt - until the home season comes round again.
In the last few weeks, there have been reports of shortages of some items (lettuces for example) because at this time of year we import these from southern Europe, and they have been experiencing unusually harsh winter weather in Spain, which has affected our supplies.
My preference is always to buy home grown, and even locally-grown stuff wherever possible, but our situation in the UK makes this difficult much of the time.
In our garden at home we have an apple tree and a pear tree, and I also grow strawberries, but I'm thinking about planting some fruit bushes also to increase the "home grown" quota.