If possible, read English books you like aloud often. Watch TV shows and movies from the United Kingdom, and listen to British music. Make friends with native speakers online and talk with them on Skype.
Basically, immerse yourself as much as possible in British culture without actually going to the UK.
I did all the same things with Arabic and Egypt instead of English and the UK, and my friends told me my pronunciation improved a lot quickly.
Do you know the book "Speaking Extra" by Mick Gammidge (publishing company: Cambridge University Press)? There are a lot of speaking lessons, each conversation topic in 3 levels. Maybe you can practise them with a skype partner.