Internship to my cv or volunteer to my English ?

Hello, I'm a French student in the third year of a master's degree. Between may and December 2021, I have 7 months free, our school expects us to go outside the country to have an internship or a relevant experience. I've done two years of  'preparatory class', so I have no work experience at the moment. However, my level in English is not enough to have a relevant internship outside the country. There are some techniques, like, doing my internship in a French enterprise in another country (so I would speak only French in this company) or to do my internship in some French speaking country like Luxembourg or Switzerland, But I don't like these techniques because I really want to have in job in London, after my graduation. Therefore I have to dramatically improve my English. So my question is: Should I do a  relevant internship or should I find a volunteer job in an association in Great Britain to really learn English? 
Will 6-7 months in London be enough to improve my English? The last time I visited London for two week seemed very useless to me. 

My expectation is to have a Spring in IB in 2022 (fourth year of master's degree) and a Summer in 2023 (I will candidate for a spring without work experience, but with better English or a better work experience without good English)