I'm on the long, meandering path to this career, over here in the States, so I can help :D First off, there is no one way to do it, though there are commonalities among people who do succeed in the industry. Also, it is ridiculously hard to succeed in, and is one of the most niche jobs in the journalism industry. It is incredibly worth noting that most film critics are middle-aged white men, and film critics hold onto their jobs until the day they die -- because it's one of the coolest jobs in the world! Generally, you're going to have to do a LOT of scut work and work your way up before securing this position, and it usually won't happen fulltime until you are in your late 30s or early 40s (unless the publication has a critic die on them, and they decide to get someone "young and hip" - but I've only seen that happen once!).
I would do A levels in English, for sure, and then some complimentary subjects, like History or a Foreign Language - humanities, really (psychology? art history?). To get into journalism in general, English would be your best subject to do at university, or journalism if they offer it. Taking some classes in film studies is a really good idea, too. In the UK, you may not need to go to university to get a job, though (in the US, you do).
Work experience is key! Do as many as you can, at as many publications as you can. Obviously you want to be a film critic, but do all sorts of things in entertainment journalism (TV, fashion, food, travel, etc. etc.!), make contacts, if you're lucky do some writing (usually you don't get to, at least not at big publications), and if you're really lucky, get a job. Usually you'll start out as an editorial assistant, which is the entry level job for journalism, and work your way up. All the while, you should try and freelance reviews, probably starting for website that needs volunteer writers (start with DVD reviews, perhaps) and build up a portfolio of clips.
At your work experiences, always be eager and gracious, and wait for an opportunity where they need something covered, and step in! Most likely you'll freelance reviews for AGES until a staff position opens up somewhere. In fact, you're not too young to start - check gumtree.co.uk and see if any websites need volunteers to write capsule reviews.
The way I've done it is a bit different, and very specific to the US education system. I was a journalism major at my university, and eventually did rotating jobs as the entertainment editor, entertainment managing and film editor of the university newspaper. At the paper we ran film reviews every week, and got all the press access to film screenings that normal publications get, so I was able to get my feet wet and develop a nice collection of clips (including celebrity interviews) early. These helped me get some good internships, including with an international film publication, and a part time job at a local paper writing bar features. I'm still working at it though, and sympathize with your goals! It's very, very hard, and I don't expect I'll achieve the end goal for some time.
When I was studying in the UK, I did several work experience placements, but not at a film publication b/c I found out how hard it is to get placements there! You might be at an advantage going to uni, trying to get into journalism at a school publication, and see if your college has connections at some places and can get you work experience placements -- my school had a whole program for that. Ask me questions anytime you want -- including some publications in London that do good work experience. As far as I recall, you can start doing them when you're 16, so you might be able to start now, and do some work in an office for a few weeks.
Answered By: clareapparent - 4/2/2008