The single most unrealistic bit in V for Vendetta was in the flashback sequence where it shows how Norsefire won the initial election - it shows a diagram of the distribution of seats in Westminster and only alots about 6 to the existing big two parties. And honestly, I don't think that's really possible. Party loyalty is very strong amongst certain sections of British society, i.e. for many people the idea of not voting Labour, or not voting Conservative, is nearly unthinkable. It's all down to our daft old class system.
No, I think the best way to see fascism arrive in Britain is for it to come through the Conservative Party. For instance, imagine a TL where the Liberals collapse basically as in OTL, but where the Labour Party fails to either a) come into existance, or b) over an electable alternative. The Tories then find themselves with in a political vacuum, and are able to expand accordingly. By this point, politics mostly occurs within the smokey back-rooms of Conservative Central Office, and not in the relative open of parliamentary politics. A group rises to prominence amongst the Tories of proto-Fascists.
Meanwhile, in Europe, the Spartacists seize control of Weimar Germany, the Republicans win the Spanish Civil War, and across the Channel, the Red Threat seems to be growing. This empowers the fascist-leaning Tories, and after holding a vote of no confidence in the sitting PM, manage to wrestle control of Parliament. Socialist politics are supressed and Britain becomes increasingly authoritarian.
So on, and so on, assuming there is a Second World War, Britain will almost certainly be on the winning side of it, thus further cementing the rule of the Party. The monopoly on power is furthered, dissent is crushed, and voila, Fascist Britain.
