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.
Interesting, average rent in London is around £1000/month, plus £44 council tax (e.g. Wandsworth), plus monthly travelcard £120, gas and electricity £45, broadband/phone £16, mobile phone £15. Leaves about £60/week for food, clothes, etc. If you do manage to find a studio flat for £700/month then that brings it up to £130/week, so you could actually go to a pub sometimes.
I'd pay 65 quid for a mothly bus pass (I didn't have to use subway on a typical day), and the rent was £65 a week plus electricity (3rd zone just outside of the 2nd, Streatham Hill) - for a bedsit, kind of a shithole, but I've seen worse. It was around 2010.
Anecdotally, salaries are lower and rent higher as compared with both NY and San Francisco. Taxes are obviously higher as well.