There are in fact three green parties in the UK, founded when the Green Party (UK) amicably split in 1990:
They are considered together in this website as they all share the same philosophy, and consequently a swing towards one green party is likely to be reflected as a swing towards them all.
The Greens got 8.6% of GB votes in 2009, which was 2.5 points more than they had achieved in 2004 or 1999, although that increase in votes didn't give them any more seats -- in all three elections they won 2 seats.
This is because there is an implied hurdle in the voting system at about 10% of the vote: if a party gets more than that, they will get seats at least proportional to their vote, but if they get a lower vote share, they will get a lower proportion of seats than votes. So for example, the Greens got 8.4% of UK votes but only 2.8% of seats.
This effect could work to the Greens' advantage if they do well: for example if they get an extra 2% of votes across the UK, they will win 6 seats, three time more than they currently have.
|East of England||141,016||8.80||0.70||0/7|