I had to pass a TOEIC test and obtain more than 780/990 in order to graduate from my engineering school in France. The first time I took the test, I got 480. Fair to say, not quite enough!
For the next year, having 2 hours of English classes a week, I started watching American TV series. From 24, to How I Met your Mother through Big Bang Theory, Dexter and honestly, so many more! A year later, I got 820 and passed. The vocabulary looked at in class started to stick into my head as I would hear uses of it in the shows!
I went from struggling to understand the teacher in class to sometimes starting episodes without subtitles and not noticing. If you don't like TV series, consider it an investment on your English: it's fun, you learn, and you are doing something towards your end goal.
To take it to the next level, I started reading articles written in English online. I am not talking about Facebook stuff, I'm talking about scientific reviews or anything that could interest you. It being about DIYing, photography or raising kids. Remember, you do not need to know everything, even the British people don't know everything... Because they are not interested in everything. I now live in Scotland and so many times has it happened to me that, I would meet someone, talk about the process for buying flat, and they don't know half the words. I knew these words because I was trying to buy one myself.
Having said all of that, there is nothing better than even a short 2 weeks trip above, by yourself. Trust yourself, you already speak better than you think!