Hard to say, as Ingsoc says. Rally around the flag like the US in 2002, or punish the government like Spain in 2004 - either is possible.
The Tories might end up squeezed either way, come to think of it: option A boosts Labour, option B probably mostly boosts the Lib Dems as the anti-war standard bearers.
To some extent it might depend on how the government handles the aftermath. Blair's obviously the master of managing a crisis for a favourable media outcome, but every time he appears he leaves himself open to being accused of his foreign policy bringing it down on us.
There's also the point that reaction to the OTL 7/7 attacks was I think influenced by the fact that the second wave two weeks later failed, whether in terms of reassuring people because they failed or worrying them that it wasn't a one-off attempt.