The Euro-Election voting system explained

The Euro Election on June 4th will be held under a weak system of Proportional Representation called the d'Hondt method.

In short, at the count, when all the votes have been spread out proportionally there will probably be a place left over for one of the 'smaller' parties. Which could mean the either the BNP or the Greens.
We know that if a Green is elected, there is no room for the BNP.

Therefore, anyone who wants to stop the BNP should vote Green.

Thanks to Roger Creagh-Osborne for the following explanation:

Under the d'Hondt proportional system each seat in order goes to the party with the highest number of votes. Then that party's votes are divided by the number of seats they have won plus one for deciding the next seat.

So suppose you vote for a major party that wins two seats out of the first five in your region, when it comes to the sixth seat your vote is only worth one third of a vote against a party that hasn't yet won a seat. Since the Greens are likely to be fighting with other smaller parties for the last seat, if you want to exclude a particular minority party your vote will be two three or four times more effective if you give it to the Greens rather than a big party.

Votes for big parties are diluted when it comes to defeating the BNP.

This means that a Green vote is far more effective at keeping out the BNP than a Con/Lab/LibDem vote.

Of course, there are many other excellent reasons for voting Green, such as, it is absolutely right to base economics on ecology, the Green New Deal will create hundreds of thousands of good jobs, the uK needs to join the rest of the democratic nations in having a Green voice in their parliaments. and a big Green vote will stimulate the other parties to clean up their act.