It is important to note, however, that in the UK the system usually used in elections is First-past-the-post, which handicaps smaller parties.
In continental Europe we are more used to proportional representation, which gives more impact to votes for smaller parties.
If you want to compare, you would have to look at e.g. European Parliament elections (nice coincidence, we’ll have them soon), which uses proportional representation EU-wide.
In the 2014 European Parliament elections in the UK, the Green Parties in the UK combined got 7,7% of the UK vote (England and Wales 6,9% + Scotland 0,7% + Northern Ireland 0,1%).