London is the capitol (and largest city) of England and United Kingdom. UNITY has it's headquarters in secret location near London, a lot of UNITY employees live in or near this city. Both Cate Archer and Mr. Smith have flats (apartments) in London.
Cate Archer and Dr. Otto Schenker were on a flight from Berlin to London when their airliner was attacked by Magnus Armstrong and paratroopers from H.A.R.M.. Just before the attack the pilot had announced that they planned to arrive in London at 6:15pm local time. The weather report showed 25 degrees centigrade and clear skies.
London on Wikipedia