Olivia has several advantages:
1. She is alive.
2. She likes Giles enough to buy a return ticket, go to the airport, put up with all the security hassles, fly thirteen hours on airline food, take a taxi to Giles' place and sleep with him then put up with all the same hassles of the return trip, although it might take a little less time if they catch the Jetstream on the way back.
3. She is the only to turn the monotony of Giles' life into cuppa tea, cuppa tea, definitely got shagged, cuppa tea.