purits, that point about a hypothetical election being decided by BNP voters is a striking one, but I do believe mainstream parties court the fringes even now, and especially now, because it's important not to let the rightwing/leftwing vote (whichever your party depends on) be splintered.
In the French election I mentioned above (the one in which Chirac was elected; he was president before Sarkozy), leftwing voters were very divided in the first round and there was a lot of apathy, and then the leftwingers got a massive shock when they realised that the "mainstream" leftwing candidate was out, and all they had left was Chirac (a conservative, who was disliked by many left-voting people) and Jean-Marie Le Pen, a fascist (effectively BNP, but much more powerful in France). In the French presidential voting system, there was a second round, so thankfully (a) leftwing-voting people had a chance to vote out Le Pen (which some did while wearing a clothespeg on their nose), and (b) rightwing-voting people who were conservative but not fascist did not end up with a fascist because some leftwingers had been apathetic and had not voted for what they really wanted/would accept.
If there were a way of expressing preferences (like AV), voters could thumb their noses at Labour/Conservative by making the party wait for their second vote - without risking a victory for a party they really don't want in.
With our current system, if you cast a protest vote, you risk allowing in the Other Party (the one which is designed not to appeal to you, since parties are currently dividing us up between themselves!), whose voters may be more disciplined.
The current system basically encourages us all to be dishonest, to ourselves and to one another: voters and politicians.