Glassdoor suggests "software developers" or "software engineers" in London earn around $60k, which would put it far below the salaries listed here.

In the absence of the COL index for the UK we can use the "London living wage" - supposedly a baseline for a minimum acceptable standard of living[1] which translates to £18k (a little under $30k). The tax paid on the amount earned in excess of this living wage is around £7k (<$11k) That would leave around $19k of savings or "discretionary spending", actually slightly better than NYC and Portland despite the low salary, but based on what is almost certainly a much less generous view of essential costs of living.

[1]Since it assumes dependents it's quite a generous minimum standard; indeed excluding tax payments from the equation I've never spent more than it whilst living in London on substantially higher income. But it's also almost certainly less generous than the COL assumptions for the US cities.