No, really, imported food can have lower CO2 emissions than locally grown:

One study by Lincoln University, in New Zealand, found that
    2,849kg of carbon dioxide is produced for every tonne of lamb raised
    in Britain, while just 688kg of the gas is released with imported
    New Zealand lamb, even after it has travelled the 11,000 miles to
    Britain. Researchers and farmers in Britain have raised doubts over
    the accuracy of the New Zealand figures, but they concede that sheep
    farming in New Zealand is more efficient than in our own country.
…
But other studies of fruit and vegetable production have revealed
    a more complex picture. Research by the centre for environmental
    strategy at Surrey University has shown that British apples are
    better for the environment during autumn and winter, but in spring
    and summer it is "greener" to import them.
Dr Llorenç Milà I Canals, of Surrey University, said: "By
    May, apples harvested in Britain have been kept in refrigerated
    storage for more than six months, which uses a lot of energy. At
    that point, it becomes better to import from New Zealand."
…

He has also found similar results for the production of lettuces,
    which showed that the energy used to produce out-of-season lettuces
    in winter in Britain was greater than importing lettuces from Spain.
    He added: "If you are producing lettuce in a heated glasshouse
    in the UK, the amount of energy you are using is huge, so in that
    case buying British produce over winter is a bad idea."

Similarly, British farmers who grow tomatoes and strawberries
    often rely on heated greenhouses to produce crops outside the short
    fruit season in Britain. Dr Adrian Williams, an agriculture expert
    from Cranfield University, in Bedfordshire, said: "If you
    produce something in an unheated greenhouse abroad or in a field,
    you make a considerable saving, as you are not having to use large
    amounts of energy heating a greenhouse. You could expect there to be
    a difference even if you allow for the transport from Spain."

Earlier this year, Mr Williams revealed that growing roses in
    Kenya produces just 17 per cent as much carbon dioxide as growing
    them in Holland. Importing beans by air from Uganda or Kenya is also
    more efficient.

Prof Edwards-Jones explained why: "In Uganda, they tend to
    have small farms that export beans. They don’t use tractors, as
    it is all done by hand, they use cow muck instead of fertiliser and
    don’t use hi-tech irrigation systems."

…

But even the method of transport is generating controversy. Some
    researchers claim shipping is better than air freight, but others
    insist that for perishable goods, packing them into a plane for a
    quick journey is better than refrigerating them on a cargo ship. Air
    freight contributes just 0.1 per cent to Britain’s carbon
    dioxide emissions.

0.1 % of CO2 emissions is caused by air freight? OK, even I have a hard time believing that number but if it’s true, then really, all these peopple wibbling on about it are severely deluded. They’re quite possibly causing more emissions shouting about it than the process itself actually generates.