People do throw terrorism around a lot these days, but remember that terrorism, like sin, doth lie in the intent. These guys fall into the Anders Brevig category of terrorists: if you're murdering someone - be it with a gun, a machete or a bomb - with the intention of triggering some kind of political or social change by provoking fear in the surviving population, then that's a terrorist action. It's also a pretty broad definition, and catches a lot of things that don't fall into the nebulous public definition of "Real terrorism" (like your aforementioned semi-political school shootings). That said, in this case, based on what we know so far, it does seem to apply - certainly it's a better fit than the Syrian government calling the rebels there terrorists or something like that.
However, I hope and expect that the UK will treat these men as what they are - criminals, to be tried and convicted by the justice system. The people to really watch are your UKIP/BNP/EDL type people. A brutal, public, political crime with a very black, very muslim, very photographed pair of assailants - this feels very much like the exact kind of thing that those kinds of people are always waiting to pounce on.
