I assume you pay tax at 40%, in which case over £14,000pa of your gross earnings is going just to pay for your travel to work. Add in say £2,000pa gross of £2.50 coffees and the junk food commuting makes you buy. Then you have the 20 odd hours per week of unpaid travel time.
I did that sum for myself a few years ago and decided it wasnt worth it. Obviously everyone's situation is different but often the London premium doesnt actually provide a better quality of life.