@ChocoTrio I am not saying London isn't expensive. But what I am saying is that depending on your circumstances, it may be more expensive to move further out, simply because there aren't accessible options in a small town. Of course if you live in Reading, work in Reading, that is definitely cheaper as Reading is a big town with many shopping options for all budgets, wide variety of housing, viable employment options within a short drive or bus ride etc. However if you live in a Home Counties town and you have to drive/take the train, that would cost. And that is a sunk cost which you can't get back unless investing in property.
my DH and I don;t earn much but we save 25%-50% of our salary in London. I don't think we would be able to do that if we paid 2 season tickets or had a car and commuted long hours everyday/had to pay parking. I don't think comfort should really be a factor because there isn't really a way of assessing that. A freehold 1000 sq feet house might be acceptable to most Brits as desirable housing but to an american, that is completely unacceptable as their houses are much bigger there (but the cost of getting a 3000 sq feet house would be prohibitive here in most desirable areas inside and outside London) .
what I am looking at is the smallest cost a couple can incur assuming both have London jobs.
One has to take a lot of these measures with a pinch of salt. For example, the stats have always measured Singapore to be the most expensive city in the world. These stats are used by expats to determine whether a place is suitable for them to move to and whether their pay would cover the comforts they are used to. However they often do not take into account the way the local population live. In Singapore, the cost of a car is always factored in despite the fact that only 25% of the population own a car and most use public transport. 78% of the population live in government subsidized housing that they own, and don't pay private rent (hence private rent isn't even an expense most people have). Alcohol is always factored in despite the fact that there is no drinking culture and most locals drink very rarely. Likewise in London, people adapt their lives so as not to incur the high costs; there is more sharing and communal living to save on space.
Of course you are right. London is an expensive city. However ex Londoners often move to places which are not much cheaper or retain jobs in London which mean that the added savings go on travel.