American accent vs. British accent? I think that you should decide on whichever is more comfortable for you and then redouble your efforts to be able to follow and derive the most benefit from your English language studies in the UK.
My guess is that your teachers will use BE in class. Native English speakers in the UK speak BE but they can also understand AE (the accent & differences in vocabulary) because TV in Britain has so many American TV programmes and lots of daily news coverage from the US.
In preparation for when you arrive in the UK, may I suggest that you tune into the BBC's 24-hour world channel on TV. Listen and enjoy BE on the BBC when you are living and studying in the UK. Avoid watching American TV programs (AE spelling of "programme") until you have made sufficient progress to be able to better understand both AE & BE in equal measure.
You should avoid mixing BE with AE and vice-versa when you are speaking and writing class assignments. Please remember that the importance of one form of English over the other only really becomes a problem in situations when you are communicating in English with another non-native speaker of English: your common tongue (lingua franca) should then be either 100% AE or 100% BE. In the UK, I would do your best to adopt BE to ensure that you are able to communicate in English with your teachers, classmates and new friends.