If this new strain of E Coli escapes into pandemic proportions then the users of London's buses and tubes will be amongst the first affected. E Coli is transmitted in exactly the same way as the Winter Vomiting Bug - in minute particles of human faeces left on poles, doors and grab-rails by people who don't wash their hands properly after using the lavatory. And London's full of 'em. Even if the source of the bug on salad crops is identified and contained, those already infected will be transmitters unless they too are isolated.
And paradoxically, this is a good opportunity for British farmers and growers to sell to the public directly rather than through the big supermarkets - the Tesco salads counter will be shunned for the next week or so as fear of Mediterranean muck takes hold. Torrential downpours recently in Extremadura have no doubt contaminated stored water with human sewage and animal manure, which has then been carelessly used to wash or water the salad crops, whilst in the South of France a record drought has seen draconian restrictions on water use.
So the answer seems to be don't boil the lettuce as long as it's British, but do wear gloves on the bus.