if it lands on time (?) and you are a UK citizen and must join the passport queue and you land and park at Gate 80 you could be out of the airport about 30 -45 minutes later depending on wind direction for landing and taxi distance.
If you go to Central after getting the ticket and assuming the train is waiting you will be there at probably 1900 hrs in the Central station then a fair walk to the area where you meet.
I suggest you meet at the Novotel Tung Chung to be sure.
CX gates close 20 minutes before departure so you need to be there by 2325 to be sure. You need to clear security and immigration queue dependent and if your flight is on a far gate (33-80) allow min 15-20 mins to get there after clearing immigration. If you allow 30 minutes to clear security and immigration you need to be at the airport by 2230 latest. The Novotel is 10 minutes in a cab away.