If EVERY BNP supporter joined Yougov, we could put our support on a par with the Lib Dems and possibly higher! On top of this we could actually raise money for the Party via Yougov as you actually get paid for participating in surveys.In short, if they don't tell YouGov that they are BNP members then the only way BNP 'over-representation' would get filtered out is by comparison with other data-sets. YouGov should be able to spot outliers.
I made a call to a friend in the polling industry and his view is that this could mean that YouGov are subsidising knuckledraggers and getting their data wrong - over-estimating BNP support (though they do, I understand, weight their conclusions on the understanding that some people don't even admit to pollsters that they vote for the BNP).
Of course the other issue is that - if the BNP have a demographic significantly different from the broad swathe of UK nationals - it is possible that YouGov's 'normalisation' would sideline BNP-members opinions.
Now, if it turned out that BNP members have a dramatically untypical British demographic, even amongst 'ethnic whites', then this would be quite telling - wouldn't it?