I work in a third sector organisation with offices in about 10 places around the country. I'd guess around 40% of staff are in the London office. They pay a London Allowance of c£6,200 to everyone. So, for someone starting off at the absolute bottom band of admin it's about 40% of their salary. For some mid-ranking like me (I'm on c60k), it's worth c10%. For more senior people it's worth less than 5%. Salaries don't go higher than c£120k, although the number of people on more than 80-90k is fewer than a dozen.
I think this is unfair. It means starting salaries at the bottom range are way higher in London that the regions (proportionately), and that as you go up the ranks you stop receiving any real allowance at all, despite all the extra expenses of London. I've raised this at work, but get met with blank stares and evil looks with everyone responding: "but the extra costs of London are the same for everyone". I just think this is nonsense, and totally unfair on people like me (plus those at the lower ends outside the capital).
What do other people think? How does your organisation do it? I think the London weighting should be a percentage of salary - somewhere between 15% and 20% would seem about right.