> I've just google image searched for the Spitalfields Market, apparently people in here believe that plums and avocado grow just outside of London (because that's what I saw in those photos).

London is only 35 miles away from the sea, how far do you think it is to a field.

As to your Van Drive: For example, Brixham (160 miles) is a popular place to get high quality seafood for the posher end of the restauraunts, this is ~3hr30. Getting fresh ingredients to london is a completely solved problem.