You've probably heard this before but the healthy maximum for losing weight is 2lb/week. However, if you haven't exercised much before, the first month or so will probably see a quicker drop in weIght. Let's say this evens out the time you can't exercise around your A Level modules - if we include a brief plateau, you can probably lose 50lbs by February half-term
But, of course, it's not as easy as it sounds. Again, this might be something you've heard already but it requires a lifestyle change, not just a temporary change or "diet". Start by incorporating cardio into your day, 2-4 days a week. This can be anything from a weekend hike to a jog to even just a walk (at least an hour long) - you can do a variety of types of exercise or just one, whatever suits you best. After two or three weeks, start doing some strength exercises - sign up to a gym and look up some weightlifting tutorials on YouTube (if you're anxious about getting "hench", don't worry at all - it's so, so hard for bodybuilders to look like they do. You won't bulk up at all) using
weights that you can comfortably lift to avoid muscle injury. Also experiment with different kinds of strength exercise - yoga, pilates, barre etc. I'd recommend eventually doing cardio 2/3 times a week and strength twice a week.
As for your diet, try to make it as healthy as possible. You don't need to cut out junk food entirely; set yourself a cheat day/meal every week if you're craving it.
Don't cut down on carbs because your energy will slowly deplete and you'll binge. Up your protein intake a little. Eat some fruit and a
lot of veggies. A good meal plan to roughly follow is the NHS couch to 5k eating plan. *edit* as BristolFresher15 says, caloric deficit is essential to losing weight, but don't go crazy counting calories or anything. Try to eat until you're 80% full at every meal because it takes 20 minutes to digest food and for your brain to feel full.
Good luck!