Don't know about good but economical colleges in London but there is one in Farnborough which is affiliated to Surrey University (i.e. certificate comes from the University), one of the top Universities of UK (no. 12, I guess, you can see the list of top universities in Times Top Universties Guide). As I was there for my undergrad, I know that it is very good college. You can use all the resources in Surrey University, including library and online database worth millions of pounds. Best of all, you will find British nepalese in the class (mostly children of ex-gurkhas who migrated to UK) and also the admin staff are very helpful.
Tuition Fees, though not cheap as compared to US colleges, is half that in the university itself and the calibre of Prof. and lecturer is very high. Mine were ex-employees of world's top investment banks, chemical companies, marketing companies, etc. some of whom taught in Surrey University as well.
The classes are relavtively small unless you go for popular courses like marketing, so you tend to get one to one attention. As the students are predominantly English from nearby villages and because the place is not a City, they were not as friendly as the ones in the Universtities that I went to in other Cities for my other qualifications but they were all right.
Overall, it is the best value for money you can get for your brother's education.
If you want to look for colleges in London, you can find the list on the website below. Those not listed are private colleges and are only as good as the private institutes in Kathmandu.
Best of Luck
http://myopeningpage.googlepages.com/listofuniversitiesinuk