If you live abroad you can still buy train tickets at United Kingdom prices. Raileasy accepts worldwide credit cards as well as UK ones, however they wont send tickets overseas so you need to select the option to collect tickets from the self-service ticket machines. You will need the same credit card that you used to make the booking to get your ticket from the machines.
If you are travelling a short distance for example, London to Oxford you can buy a ticket at the station and just hop on. For longer distances such as, London to Glasgow you can also buy tickets on the day. Advance booking is not needed on any United Kingdom train. The biggest advantage of booking online prior to your journey is that you can find cheap fares