I'm going to make a suggestion that i know many here will flame me for but i think it needs to be said.
Most 'white Brits' are opposed to immigration.
Do you mean "most white British people" or are you extrapolating a group of people you know to some kind of national scale?
I think we all know many people who 'don't like the way the country is going'.
True, but that doesn't mean that "discontent = a vote for the BNP", and "the way the country is going" isn't as wrapped up in immigration as the media would wish us to believe.
Looking at the demographics i think we have to accept that the non-white population is going to continue grow for the forseeable future and the white population will decrease.
Except that, of course, you can project no such thing.
Take a look at demographic forecasts from 10. 15 and 20 years ago of the present. The similarity they share is an over-estimation of "non-white" ethnic minority population growth. This is partly because they didn't factor in intra-EU relocation, and partly because they didn't predict a slow upswing in "native" birthrate alongside a decline in the family size of 2nd and 3rd generation British from ethnic minority backgrounds.
Demographics are blunt tools. Like polls, the same information can be represented to say many different things.
I just think a lot of racist whites are going to resent this because they believe that 'it's their country' and i can honestly see the BNP vote rocketing as this transformation becomes more obvious. Some people will simply feel they have no choice but to vote BNP.
I can see the BNP picking up votes, but "rocketing" is hardly the word I'd use, and they themselves will be content with a slow, uncontroversial building of membership, which will allow them to further represent themselves as a legitimate "protest vote"
So how do we go about preventing this seemingly inevitable scenario?
We watch, we wait, and we make sure that the truth is never submerged by the fiction.