"You mean cash/paper ticket from an attendant at a window?"
In this case you can. For context, note that there are no ticket windows at most tube stations, only machines. The station at T5 is unusual in that it still has a manned window.
Extra context. Buying cash / paper tickets for travel within London is a minority sport. The vast majority of travellers don't do it. The overwhelming majority of passengers use oyster (or a contactless bank card or Apple/Android pay - which can act like an oyster without any need to actually possess an Oyster card).