It's a country or (arguably) a nationality rather than a true description of food. China has 3 times the population of Europe and a greater landmass with an equivalently diverse set of cuisines; after all no one would compare Polish and Italian food - in fact food from Puglia and from Piedmont is dramatically different.
Traditionally, "Chinese" food in the UK was Cantonese in origin (as that is where British traders went to trade) with some Pekingese influences. Now we have access to Shanghainese, Szechuanese, the food of Hainan etc.
What do you like: light steamed dishes with an abundance of fish and poultry, spicy, oily slow cooked meats or something else.
It may not be the answer that you want but it is an answer!
Weng Wah House, Haverstock Hill, NW3. Okay, not SE1, but you asked where's the best, and in my opinion, (although I'm no connoisseur) I found this restaurant to be the best as far as I'm concerned.
Closer to home, Noodle Time in Nelson Road, Greenwich SE10 is worth an honourable mention.