You are probably best hiring a portable concentrator, providing you can use pulse flow delivery system.
I would recommend a machine that goes to 3lpm. There is no guarantee that the settings are equall to lpm.
Never seen any medical oxygen cylinders for hire in U.K.
You will need a copy of your oxygen prescription from your respitory nurse, or the person who carried out the test.
information from easyJet.
Oxygen concentrators (either mains or battery powered) are permitted on board - you will need a medical certificate confirming the concentrator is required for medical reasons and that you are fit to fly. Batteries will have to be used if you need oxygen on board (be sure the batteries have enough power for the duration of the trip, including possible delays).