there's no legal framework to delay the election though - and like you say, something similar (actually something that probably got a much shocked reaction than this, which is a sad indictment on how used we are to these things) didn't delay an election in the past so delaying this one would be setting a precedent that I honestly would rather that we didn't set - the only reason to start delaying elections in my eyes should be something along the lines of a World War; I don't see why we should let terrorists start influencing when we hold our elections...
Yeah, I agree, a terrorist attack is not enough to cancel an election. Elections should only be cancelled in case of war, a coup d'etat, significant revolts that make going to the polls impossible (and that warrant an emergency state if Britain has those) and the like.
As someone said above the election was not cancelled in October 1974 after an IRA bombing. (and I guess there were more IRA bombings during the campaigns back in the day) Another example is the 2004 Madrid attacks (3 days before the election, election was not cancelled). Those were also larger than the recient ones in Britain in terms of dead and wounded people.
Not to mention, how would the election be cancelled in the first place? Iirc parliament is dissolved so there's no one to pass a bill prolonging that parliament's life. I guess the queen or the prime minister could use some of their emergency powers but I'm not sure exactly how.