I see you are a CO flyer. You will have to pay them leaving LHR going to Africa, since they will be built into your ZED fare. The good news is on the way home if you connect back to CO, they will waive the dept. taxes if you spend less than 24 hrs in LHR. Save your OAL boarding pass and send proof of connection to the employee travel office within a certain time frame and they won't payroll deduct.