This is such a hard question to answer, because what’s “good” for one person can be so different to another. I found this out the hard way when I asked the same question about Taipei and received some crazy responses from people who either don’t know the difference between “good” and “excellent” or who have just gotten used to leading very expensive lifestyles.
That said, I personally think £50K is a perfectly “good” salary that would allow you to live reasonably comfortably in a further out area and put aside some money every month, even with the cost of living being what it currently is. Anything above this is in the “high” tax bracket, so this salary is well above “average” in terms of what normal people are actually paid.
I think a lot of people will chime in and say that you “need a six-figure salary to be able to live in London”. I’ve heard this so many times, but I can only imagine that the people who say that expect non-London lifestyles in London, for example living close to the centre and renting their own apartment without sharing with anyone else. If you have those kinds of unrealistic expectations about London then, yes, you’d need to earn well above £100K.