No sense in arguing over taste, and perhaps I am a bit of an alarmist, yet I still must insist that general proximity of farms to mega cities is problematic. Even in places like Kentucky, that has only been recently colonized by masses of industrial humans, I have found issues with the quality of animals raised near the larger cities or industrial areas. We have a lot of dirty coal power plants, vast conventional farmlands, over grazed pastures and issues with contaminated water runoff.
Though I have no degree of certainty on the matter, to me it just doesn't seem feasible that there would be any Land within 50 miles the city of London that would be considered pristine by my own standards.( Does anyone here know of any truly ideal sources, from farms close to London?)
Ive been using Google maps to investigate farms before actually driving out to inspect in person, and if I see a farm in the middle of a major big agra center, or near other major industrial, or urban activity, then I will try to look elsewhere.