When can we vote different government? (UK)?

Hi well I turned 18 making me legal to vote, but when is the next voting, I don't like the current government in at the moment so I want to vote UKIP don't judge me also what is the chances UKIP could be in power? I am voting for them because I like there policies and how immigration is crowding our country I don't want Immigration to stop like BNP i just want them to reduce it which the current people don't seem to care about! I heard UKIP are getting stronger.