So are you asking for advice on whether to keep your BTEC or switch over to A Levels, or are you asking what subjects to do to keep your options open?
The only real way to be sure if you want to do film or not is if you already studying the degree. Whilst you can get input from anyone who did or is doing the degree, their personal experience will be different from yours.
To my knowledge, film degrees generally accept BTECs and A Levels in any subject (i.e. no required subjects). In other words, you can do the degree so long you have the right grades for your Level 3 qualification. You're not at a loss in picking a subject that's not in film studies, and having a BTEC/A Level in film studies is not required nor put you at an advantage in your uni application.
If you do pick A Levels, you would be open to a wider range of degrees, especially the academic ones that require A Levels over BTECs. However, this would depend more on the subjects you pick and the degree that you pick. If the sort of degrees you are going for would accept BTECs in any subject, you're not really putting yourself in much of an advantage.
Having said that, if the degree course that you want to study has required subjects and accept BTECs, it's likely they would ask you for the A Level subject alongside a specific BTEC e.g. if you want to study a degree in life sciences they normally ask for Biology and/or Chemistry as required subjects for A Levels and the BTEC they would only accept is Applied Sciences, but they will usually ask for the Biology and/or Chemistry A Level alongside the BTEC.
Normally, the appropriate steps to choosing your degree and qualification is to find out what you want to do after your degree. Since you don't know what you want to do, I would look at the sort of subjects or possible career areas that kind of interest you.
If you want the bog standard response of which subjects would keep your options open the most allowing you to go into the most degrees (provided you get the grades, etc.), then you would be looking at the 3 sciences + maths. Outside of STEM, A Level Maths allows you to go into the pickier finance degrees, actuarial science, and economics.
Required subjects on certain degrees can include: art, literature, history, geography, classics, and languages; usually for their respective subjects. Having said that, art and literature are not necessarily required for degrees from mid-tier to low tier universities, and you don't necessarily need geography for human geography degrees (if there is a physical geography component, you would need the A Level).
If the above subjects are not areas that you have any interest in, then you don't need to worry about which subjects you would need to pick. If the degree course that you want to apply for does not need specific A Level subjects and accept any BTEC, then there's not much advantage in switching over to A Levels (other than to say you have A Levels as opposed to a BTEC in your application).
The sort of degrees that are required for specific jobs then to be in healthcare/life sciences, academia, or education. Those areas that would require degree level education that can be achieved through professional qualifications and apprenticeships tend to be in accounting, actuary, engineering, medicine, economics, nursing, architecture, law.
Most things in healthcare or life sciences would require A Levels in biology and chemistry. Engineering, actuary, and economics would likely need A Level Maths (with very likely Physics on top for engineering). Accounting, nursing, law, and architecture won't require specific subjects (some of these would accept BTECs).
If you want to go into government services, your BTEC should be fine, and they generally don't require specific subjects. I would check in the specific area that you want to work in for their entry requirements to be sure though. A Levels are accepted, but they generally don't specify any required subjects.
If you can give a rough idea of which industries or subjects you're interested in, I would be able to at least look into what you should do or which subjects you should pick.