It is definitely quite late in the year, but I know someone who swapped subjects around a similar time last year, so depending on the school it is possible. I think you will have to consider the sheer volume of content you are going to have to catch up on, because you've got almost half a year missed in whatever you swap into, and for something like bio/chem especially it is a lot of catchup, especially if your school will mostly have mocks at the end of this year that act to dictate your predicted grades.
With what you've said about eng lit, I'm assuming you are taking OCR, and while the mark scheme states that 50% of your marks come from context - AO3, but this that mean it should be 50% of the actual content in your essay. The real goal is ensuring all the analysis all relates to the text, as the context plays such an important role in why something was written --> was it reflecting events at the time, was the author feeling they had something to say, does the writing reflect their lived experiences, are there echoes of previous/future texts in the writing? Something my teachers have made sure to highlight is that context isn't only historical, it can also be biographical (relating to the author/writer) and literary (other texts and the standards of literature at the time), and with practice it does become increasingly natural to include in your essay, its just a matter of having links with each of your broad analysis points that tie it to one of these aspects where possible.
Fundamentally though, it's your choice, and if you are committed and certain you would be willing to put in the work, and try to attain higher in bio/chem etc. then talk to your school and see if it's possible
And potentially keep in mind future prospects, what are you trying to do? --> would english be more useful?