You can be anywhere in the UK to apply online via the Thai Embassy, London. Staying with a friend, provide the friends address and form of ID/letter from friend detailing your stay. UK Thai embassy is ok with a final balance of 500K THB, not needed to provide 6 months of statements
Best to stay until approved in case of having to prove your presence in Cambodia or the rare chance of even being invited for a face-to-face interview.