If possible, you should try to do the PCR test for China within the 48 hours of your DOH departure just to save yourself some trouble at transit point in DOH. Not sure how long your journey will be, so this could be tough. The old rule used to be 48 hours before the last flight to China if connecting. New notice on their embassy website is just as confusing as the old one.....I am doing DXB-DOH-HKG then land into ShenZhen next week, plan to do an express PCR on the day of my DXB departure, so everything is within the 48 hour window.

For the return, I don't know about UK, but US used to take PCR, Video supervised test or Letter of recovery (infected in the past 90 days). what agency is in charge in UK? should check with them. Also, not sure what city you are going to, but getting a PCR test in any of the major cities in China is super easy and super cheap at all major hospitals, right now is about 15 RMB (2.5 USD) and result in about 12 hours, result can be accessed via WeChat or a Kiosk machine at the hospital.