Hi Lizzie I'm currently in a similar position where I am looking to go to film school after my final year of highschool 6th year (in Scotland) and while I don't have the "answers" I thought I could share what I've learned so far. I've seen similar from universities recommending or requiring a portfolio to get into the program I've heard this both online and at open days. I believe it is necessary especially if you're going up against candidates who have a portfolio proving their desire to get in or at least their current skill level. I've heard of people getting into film schools with just script-based portfolios without any films. Hence, it's clear that just having something and probably a mixture of a portfolio will significantly aid your chances of getting in especially if the course is competitive.
With your question regarding if you're able to utilise your A-level photography course work although I am at this time unsure (and perhaps can answer this after visiting another university open day and talking to the professors) 🤞 at the moment I wouldn't see why not especially if your course work shows the best of your ability. If you know you got a good grade in this course, it links together well and tells a story or reveals your skill level through techniques, or editing I would definitely think it would be a good move to use your coursework in your portfolio especially if you don't know where to start.
At the time of me writing this I am still searching for the answer to your third question 😅 and perhaps I will be able to respond again after prompting a professor at an open day. 🤞
From what I've read online and the minimal advice I've seen I've come up with the following conclusions: Create a good diverse (scripts, photos, films) portfolio to send to the film schools you are applying to which shows the best of your work at that time. And my personal uneducated advice do similar to me and go to open days at the universities you wenat to go to if you haven't yet seen if you actually want to go there, ask professors at booths or talks for advice they have the best answers, not me.
Good luck and if I find out any new 'key' piece of information I'll let you know.