A dictionary which happens to be a very respected publication of the English language, I think they know a bit more than you. You'll find that definition in most standard Political Dictionaries too where you might want to look up the meaning of single issue political party which is what the...
Think you need to understand what the definition of a Pressure Group is.
"an interest group organized to influence public and especially government policy but not to elect candidates to office"
http://www.merriam-webster.com/dictionary/pressure%20group
UKIP has always been a political party...