UK citizen would like to apply for tourist visa

Dear Sir
I am a UK citizen living in Cambodia, and I would like to visit Beijing for one week. I would like to know if it is possible to obtain a tourist visa from the embassy in Phnom Penh in just one day. If possible I would like to come to the embassy and get the visa straight away, as I don't have much time. Is this possible? If so, how much would it cost? Many thanks.