Voting Today

This seemed to sum the thing up quite well for me:

Greens were generally above BNP in all areas in the last European election. This meant that the BNP wouldn't get assigned a seat until the Greens did first.

When parties get a seat, the votes required for that seat are subtracted from their total, and they have to hit the target again. In the question of a Big party vs a minor party, theoretically voting for the bigger party has the possibility of wasting hundreds of thousands of votes after it gets its first seat. There is no guarantee that voting for the big party will stop a smaller party getting the next seat. Voting for someone like the Greens however, is more likely to secure a seat (especially in areas where they missed out on one last election) as long as their party gets more votes than one you don't want to win (aka BNP).