As sddsdean pointed out you can get it through the Embassy in London, look it up in the yellow pages online or you can try their website, just type in chinese embassy london into a reliable search engine. Their information is slightly out of date on the website but only on a few minor details, they'll ask you for an itinerary with your application, but you don't worry about, no one keeps tabs on you when you get there (although they insist on seeing your passport when you book into hotels and getting your details). Watch the charges, the visa doesn't cost that much but even though I had someone pick it up from the Embassy to avoid the extra for posting, they charged extra when he got there anyway. China is an absolutely beautiful country, I hope you have a really good time.