Becaroo, I completely agree and I try to buy local when I can but sadly it comes at a premium (stupid, when you think how much less it costs to transport than green beans from Kenya etc). I used to have a veg box, but had to stop as it was too expensive and only lasted a few days. If money weren't a concern, then I'd go back in a flash.
Actually, Aldi are pretty good for using British suppliers (British Cox apples, British spuds, British asparagus, British swede and even carrots with fronds sometimes). And they do fairtrade bananas at a reasonable price. Think they're the only ft produce they do sell though.
Local markets are worth a visit too, if there's one nearby? Oh, and I recently got LOADS of Jerusalem artichokes, freshly dug, from a local lady on freecycle, so it's worth keeping an eye on that. Our local group actually has surplus fruit/veg on quite a lot.
It's none of the more exciting stuff, but, tbh, things like kiwis, mangos etc aren't going to come from the UK wherever you choose to shop.
Have you got space in your garden to grow a few veg/fruit? Strawberry plants are very rewarding and you can't go wrong with salad leaves and courgettes.
I wish I could buy more 'ethical' produce, but I just do what I can, and that eases my conscience.