I'm British, in Beijing, need to leave China to renew visa. Can I do it quickly from Tokyo?

I have extended my Chinese tourist visa as much as I can from within China so need to leave. Looking at flights, its the same price more or less in October to fly to Hong Kong, Seoul and Tokyo. I've already been to the first two so thought I'd treat myself with a trip to Tokyo! But is it possible to get a same day visa in Tokyo? If not what's the quickest I can get an L visa back into China? Should I go direct to embassy or to an agent? Thanks!