1) The weather will be deadly cold, winter clothes are usually overpriced in Brazil,it might be a good idea to buy a winter coat in London instead. But you might want to take whatever you have already got that's warm and comfy.
2) Buy an oyster card to get around as it's cheaper.
Couchsurfing.or to make friends =), go for the London groups and u'll find lots to do
http://www.youngandpoor.co.uk/ - to find cheap things to do or
even
http://www.timeout.com/london/ or
http://www.smartsave.com/uk for discounts
Whenever you're out, never pay for water, always order tap water....it's free and same
as mineral.
When going shopping even it's for food ask whether you can get student discount , I've heard that at McDonalds if you buy a burger you get the second one for free if you've got a student card or a cheap deal sort of like that... but not sure.
3) Report back to us whether you got by with your English =), but you should be fine,specially if you've got good English skills. London's got tons of foreigners and it's more than common to bump into people with different accents and even bad shaky English skills....feel confident and go for it! Pretty much everyone there is used to it!