You can try to get a tourist visa to China in Chiang Mai, but it is up to the final decision of visa officer. Generally speaking, it is not hard for Uk citizens to get a China tourist visa in Thailand. Normally the duration of stay of a "L" visa is 30 days, if your estimated stay in China is longer than 30 days, please make it clear in the application form and ask for the Visa Officer's approval when you submit your application..

The visa fee for Uk applicants is higher, I don't know the amount in Bahts, the fee is about 469 RMB for single-entry visa.