I've been reading transcripts of the Stalin/Mao meetings of 1949-50 (here's an online copy) and one comment in particular jumps out. While discussing Western reactions to the PRC victory, Mao states that:
Comrade Mao Zedong: Several countries, especially Britain, are actively campaigning to recognize the People's Republic of China. However, we believe that we should not rush to be recognized. We must first bring about order to the country, strengthen our position, and then we can talk to foreign imperialists.
And, indeed, Wikipedia tells me that Britain did attempt to recognise the PRC quite quickly (but was refused).
So why was London pushing to recognise the PRC? Washington wouldn't formally do so until 1979 - were there specifically British interests at play? And was the Chinese rejection of this recognition merely symbolic?
I believe the biggest reason was that the Brits still held onto Hong Kong. The belief that recognizing the PRC may ease tensions between the two nations and prevent or at least postpone any attempts the Chinese would make at reclaiming it. The empire was dying at this point, still recovering from the war, and dealing with various problems in its other colonies as well so they weren't exactly in strong position to defend it. Of course I'm no expert on them topic and am just going off my knowledge of diplomacy so please take this answer with a grain of salt. Although this would seem to be the most logical conclusion.