In order to improve pronunciation, I highly recommend you learn the IPA chart first. Then, since you're going to the UK, try watching British movies or series. While you're watching them, pause the video and mimic the actors, try to repeat what they're saying and copy their accent and pronunciation, do it several times until you pronounce it correctly. Be persistent! At the beginning it may take a while, but if you don't give up you'll get the hang of it quickly! And if you find a word you don't recognise or understand, DON'T skip it! Write it down! And check the meaning and pronunciation on the dictionary. It would be very useful to have a notebook with you, or a note app installed on your device to collect all the new vocabulary you're learning. Oh, and remember to review them after some time! Try to think of yourself as an actor and do your best performance! Turn on the English subtitles if you need them or download srt files, some websites also offer the scripts. Lastly, you can try reading out loud a book or any text you find on the internet, make sure to have a dictionary with you, and literally look for every word you're not sure how to pronounce. That's why I recommend you learn the IPA chart, you'll get familiarised with all the existing sounds in the English language. Do it constantly and you'll improve a lot! Good luck!