In the euro elections you vote for a party list, and the number of votes determines how many on that list get in, if any.

Some seats are going to go to the minority parties which do best. Tactical voting takes on a different shape in an election like that.

The hope, and a realistic hope, is that the BNP is going to be beaten out by other minority parties, such as the Greens - or even UKIP, which may reasonably be acused of xenophobia, but isn't racist in the BNP style, as evidenced by the fact that some of their candidates are Black or Asian. (And to be fair, UKIP does in fact put up candidates against the BNP, and may well have cost them some victories in council elections.)