I'm pretty sure schools can't force you to drop a subject or transfer.
They don't have the right to. They recommend and it can seem like you're forced to drop and transfer, but legally they're not allowed.
This may well be the case. But may I suggest Bluee that you (preferably with your parent or parents) make an urgent appointment (doesn't help much - being holidays now) with your Head of English, and ever so diplomatically and politely, asked to be allowed to continue with your Advanced. Maybe you can give a good reason/excuse for your previous poor performance, e.g. you were spending too much time on your other subjects, and that
you will now make amends.
Don't ever use a confrontational approach, unless you can afford a very good lawyer to fight your case - in which case, you lose even if you win, cause they will "fix" you later (a kind of Pyrrhic victory). Nobody likes being challenged.
Of course, during the year, your teacher should have (has he/she?) given you some caution/warning that you had to buck up in your work, "or else". From what you originally indicated, you had no prior warning.