For me, 5 food miles or 40 food miles make little difference. It SHOULD mean fresher, which we'd all go for, but it's not always the case. Sometimes the mistake is in dealing in large consignments, rather than getting (say) 25 apples today and 15 tomorrow. Better to get something guaranteed to be fresh and tasty, than a more reliable selection of old and tired.
We're lucky where we are, that there are a large number of local farm shops, but I do prefer the ones where they're outlets solely for their own produce (or that of neighbours), rather than the supermarket-a-likes with little local produce and most of the same crud that the supermarkets stock. The real issue, is the 000's of miles for bland fruit & veg, when there are tasty seasonal versions that really ought to be the staples on which base our shopping and cooking.
So, in principle I'm for the concept of flagging food miles, but recognise with the criticisms that in itself it doesn't define the quality of what you get.
Drink coffee, do stupid things faster