UK citizen flying into Hong Kong. Do I need a China visa and how much does it cost please?

UK passport holders can stay in Hong kong as long as 6 months without a hong Kong visa. But once you travel to the mainland China, you need a Chian visa.
As you'll travel to the mainland a few times, you'd better apply for a multiple-entry visa in UK. A multiple-entry China visa with 6 month validity needs GBP 90. FYI: https://www.travelchinaguide.com/embassy/uk/visa-application-procedure.htm