Your confusion based on the 3 colleges can be solved the best way if you visit them on campus. Each of the colleges provides courses from different universities from UK. On top of it, some of the colleges provide courses not available on the other. For a better judgment first decide on the specialization you will be interested in, whether it is provided by the said college, and how far are you willing to go for the college (distancewise) because some of the colleges are in the center while others are farther than the main city.
So as i said, first decide on the specialization that you want to study and then make a visit to the college and if possible try to talk to the students who are studying there while you visit them.
There are other college as well. Like Softwarica, Islington and others provides you with a British degree. If you want to study in Nepal itself there are colleges like St. Xaviers Maitighar, Prime college and not to forget Deerwalk college. I would also suggest you to visit this link to take a look at Top 25 IT colleges of Nepal according to the Everestlink 2012.
As per your second question, NAMI offers Bsc(Hons) Environmental Science, BSc(Hons) Computing, BBA and MBA.
Wanting to study IT from Nepal is a great idea. But it is not a great idea to study at any college that provides IT education. Always aim at the best college that provides the education that you want to study. There are few great colleges in Kathmandu that offer Computing degree in Bachelors. They are NAMI, The British College, and many more. I would recommend you NAMI college not because I study here but also because I have experienced the teaching style here. It is different that many college in Nepal. Its partners with University of Northampton UK. So if you join this college you can achieve British while staying in Nepal.