Where are you from? You have to write an SOP showing ties to your home country, like family, bank, etc. To get the full reason why you were rejected apply for CAIPS notes. But if you didn't include an SOP, that is a big reason why you were rejected. Again, what countries have you traveled to? Are you legally in London? Why are you applying for a visitor visa? You have to convince the CIC that you will go back to your home country after your visit.