check this site:
http://www.britainusa.com/visas/visas.aspI just put through my details and I guess that i was going for the same visa I got a year ago, i WOULD NOW need prior entry clearance. I guess that detail changed, which is a drag, because it was so easy before.
I guess some questions for your case would be - How long is the course you intend to study?
How much are the fees and what to you anticipate housing costs to be?
Add on to that an estimate of transportation costs (as a student in London you can get discount transportation tickets - I think its like 8 pounds a week for a bus pass) and give yourself a bit leftover for food and other expenses. They add up quickly, especially in London. I fully exhausted my loan and my savings in that year!
They just want to make sure you are able to cover your bases financially.
The big expenses are student fees, housing, transport, and food. If you are getting help from your family, you can probably provide the Consulate with details of your family's finances, such as a few months of bank statements.