Why now?...on the eve of British elections?
Perhaps because ISIS wants the Torys to win...if the Torys win, they will continue on their plans to privatise the NHS, cut or privatise all social programs, further restrict or initiate anti-Muslim actions...in other words, create conditions that will lead to an increase in social conflict and disaffection, particularly among Muslim youth...leading to more ISIS recruits to attack in Britain and/or abroad.
ISIS is viscious and brutal, but do not for a moment think they are not strategic thinkers with long range plans. Fanatics, yes...but stupid? No.