Dear Joanna,
Like Stuart, It is some time since I was an EAL HOD, however, I completely agree with your desire to remain flexible and responsive to the mainstream. Stuart has made many good suggestions about enlisting mainstream support, but this may also depend on what you can do for them! Here are some more ideas to support your position.
Do you have detailed provision maps demonstrating the expected coverage/outcomes of each sequence of lessons for specific cohorts of pupils and specific subjects? E.g. for year 7 proficiency A/B who have missed some years of Primary Education would also need language for Maths and some coverage of the Primary Maths curriculum. I have an old SOW I can share for this.
Can you also provide teaching/ support for all core subjects as well as English at KS4? Maths and Science are just as important and are an entitlement for all. Be clear and transparent about the difference between the Step up English and the KS4 option (which ideally should support other GCSE subjects) Ensure that all English language teaching is in the service of the curriculum and explain to your leaders what Content Language integrated Learning ( CLIL) is. I certainly provided a scheme of work for my KS4 option class which included a lot of Science language work (as I was also supporting the mainstream lessons)
But if there is just one of you, don't try and cover it all - maybe work with one department/subject each term to build good working relationships across the school.
Do get in touch directly if you'd like to see some of my (slightly dated!) SOWs and provision map ideas.
Catharine