The local Ulama can answer your question better than anyone else. Determining whether England is Darul Harb or not depends on many facts and those facts can truly be analyzed by the local scholars.
Therefore I advise you to consult an authentic local scholar for your answer.
And Allah knows best.
Mufti Ikram ul Haq
Darul Ifta of Rhode Island,
13 Rabi-ul Awwal 1431/ February 25 2010