Ok, so I'm 49kg:( I try to diet and then I have to ruin it at the end of the day by overindulging. I don't want to lose a bit of belly fat because I want to look good, it isn't like that - I just want to be healthy, because I used to get acid reflux but I don't anymore, but I'm just worried about how much fat I'm actually having and I don't realise. It's really hard for me to fit exercise in, but I walk for about 30 minutes with the dog on Saturday+Sunday as well as horse riding for 30 minutes on a Saturday, I walk for about 10 minutes every Monday to Friday morning, and that's about it. My diet plan is;

Breakfast:
-Mixed berries(mainly blueberries and strawberries) with an Actimel
-Or chocolate shreddies with semi skimmed milk

Snack at 11:
-Any fruit

Lunch:
-Brown bread sandwich with chicken slices, and salad
-Sunbites

Snack(4pm)
-Fruit or nothing

Dinner - whatever is cooked for me but in a small portion

Will this work? Please suggest whatever would work for me!
P.S: I can't join a gym or any fitness clubs because there are none in my area and school ones wouldn't fit into my revision schedule.
Thank you:)

Serious 49 Kg for a 12 years old is perfectly fine.
You are still growing so you do NOT need to lose anything.