I guess you have to add to the costs of rental outside of London with things like train and travel but that is usually tempered by larger properties and quality of life. I used to live in Lower Woodford near Salisbury. I would get up very early, cycle to Salisbury Station, get the 5.01 am and be in London for 7.30 am the onto The Drain to be at work for 8. am. And then repeat on the return.
I slept on the train coming into London although the commute back was sometimes iffy. I did this as I paid £250 a month rent (shared house), had some great mates and it is a stunning place to live.
It is not for all though. My BiL used to live near Bourne in Lincolnshire and travel in every day, it nearly killed him.
I know a few people who live outside London but stay in cheap accommodation/spare rooms Mon through Thursday. Gets best of both worlds.