... or it could just be that people have seen 'the left' in action in government for a number of years now, decided it isn't working, and are looking at the alternatives.
The success of a political party is based purely on addressing the needs of the electorate in persuading them to vote for them. Nothing more, nothing less. A lot of it may be down to 'spin' but most of it will be their policies.
As you seem to be concentrating on the BNP, a lot of people do see a variety of problems with the amount of immigration into the UK. Whilst "send 'em all 'ome" is not necessarily the best solution, the other extreme of "let anyone in regardless" is not the solution either. But what are the mainstream parties offering as their solutions?
You do have to also remember that 'the left' is as much part of the problem; anyone talking about immigration immediately gets branded a racist, or similar, so people are now afraid to even talk about the issue, and when people are afraid to talk about the issue in normal conversation, that's when the extremes tend to come out because they aren't afraid to talk about it - and "hmm they're saying what I'm thinking" is how people respond.
The success of the BNP is nothing to do with big budgets, slick spin doctors, or even necessarily their policies - I'd say it's more down to mainstream politics losing touch with the electorate and being unable or unwilling to come up with a solution to counteract it.
In fact, you could even say that the rise of the right is the result of the failure of the left through years of political correctness and fighting for an 'open doors' kind of policy. Therefore, perhaps those on the political left should be putting more effort into seeing how to make their point of view more accepted by the mainstream, rather than attacking and attempting to silence those who are, after all, only operating within the same democratic system of freedom of speech and thought as you are.
Or are you not pro-democracy?