On recent courses I have taught people from France, Italy, Norway, India, Iran, Spain etc. A lot of people start out with some struggles with the language, but it becomes a lot smoother after 3 months in the classroom. As Oxford Blue says, a certain standard is needed, but your English will get better and everyone in the classroom is learning new words and new uses for sords anyway!
If you are really concerned there are at least 2 language schools in Bournemouth that specialise in teaching English to people going into aviation. There must be others elsewhere.