Hey bro! If you are really really good in maths go ahead and do maths for ALs.(EVEN IF YOU ARE REALLY GOOD DONT DO A DEGREE AND ALS TOGETHER, YOU WILL MESS UP THE BOTH) Otherwise go to IIT and do the foundation course there. Then you can do a full time B.Eng(Hons) Degree. I think that you are 16 years old right now. By 17 you can finish the foundation, by 21 you are a graduate with an engineering degree. Then you are ahead of all of the people of the same age. I'm telling you this because I regret that I couldn't do it. But my friend did it and he is working in Cambio as a senior software engineer and I'm still in my 3rd year.
When it comes to choosing a place for higher education you can't really take short cuts. You have to pay a certain amount of money and get your degree from a recognized university(If you pass ALs and go to local UNIs, All of them are equally good and there are qualified lecturers but private field is different). It's good if it's recognized worldwide. IT field is not only limited to Sri Lanka you can work anywhere in the world without having to do any other courses if you choose wisely.
When it comes to IT if you are really good and passionate about it, it's you are the person who decide your salary. Not the company. Even though I'm still a student, I make money online by doing small projects. When I made my first 1000$ I felt proud because I made money out of thin air

. It's good for our country too right? According to my vision, a small country like us should sell our services to the outside world. That way we can easily become a developed country.
It's your life anyway. Whatever we tell you, finally you have to take the decision. However you seems to be an intelligent guy who thinks ahead. That's why I posted this comment just for you. Wish you good luck for your future!
FYI:
If you have no money to do a degree and if you have simple 3 passes from science stream or 3c passes from any other streams, some private unis like iit is giving you free degrees as well. They will hook you up with a company. There you will have to learn and work in week days and on week ends you can do your degree. The company is paying for your degree and you can get a salary around 15k as well.