Hi, I am a british citizen, want to travel to china as tourist. Do i have to get a visa?

You need to apply for a L- visa to travel to China. Yes, you can apply the tourism visa ( L visa) in Sri Lanka. But I cannot guarantee you will get it successfully. I think you'd better get it in your home country, just in case of any trouble.
Requirements for applying a tourism visa:
1. A valid passport and a copy of its information page. The passport should be valid for at least 6 months with a blank visa page.
2. Completed and truly filled Visa Application Forms (Form V2011A and Form V2011B); a recent-taken passport photo, which is glued or stapled on the application form; if the applicant has a Chinese name, the Chinese name should be filled in.
You should also fill in the visa application form ( Form V2011B) if applying in Sri Lanka.
3.Other deemed necessary documents by the embassary.