Regarding the Green 15% poll, there's brief analysis of it at politicalbetting.com
"We?ve had a Euro poll from ComRes which has been commissioned by the Green Party - who come out rather well. The shares are - CON 24%: LAB 22%: LDEM 14%, UKIP 17%: Green 15%: BNP 2%. The poll was not past vote weighted which generally means that the sample would not have been politically balanced and skewed quite considerably towards Labour. It is for this reason that I?m not attaching much importance to it.
This is the sort of polling that they had at the 1992 general election"
I am always extremely suspicious of polling showing the commisioner in a very positive light.
I think you'll find more reliable numbers at a bookmakers - sportingindex have vote percentage spreads, but only during working hours, so not up at the moment.
It's worth bearing in mind of course that if the Greens really ARE doing as well as this poll suggests, then voting for them won't keep the BNP out, because they'll comfortably have at least one seat and if the BNP are fighting to get the last seat it could be against the Lib Dems, Tories or Labour.
If enough people vote BNP, then they'll be elected, simple as.
Personally I'd just vote for your preferred party....