thinking about comparisons, in SF the average 1B is $3300 https://www.zillow.com/rental-manager/market-trends/san-fran...
In NYC it is similar.
SF's challenge is that the business distract is split across 2 areas (SF, SV) 50mi apart, with extremely sparse public transit in SV. Everything is doable, just be prepared for $100 Uber bills as you go between meetings.
In NY the business district is thankfully mostly centralized. However, poor commuter train service outside of Manhattan makes everything more expensive as there is insatiable appetite in central areas to avoid the commuter trains.
The public transportation point is definitely key: London is just so unspeakably large spatially and it's all more or less well-connected that there isn't the same scarcity of commutable apartments as in NYC/SF. It wasn't uncommon for older colleagues to even commute in from Kent or elsewhere in the English countryside -- and often their morning train wasn't much longer than my own.
London is a very expensive city, and the median income is actually very low compared to it. There is a high concentration of people who are very well off, but everyone else is struggling.
I wonder what's you definition of struggling, especially "everyone else"
High rents encourages startups to be founded by those who haven't coupled up and who choose to live together.
A company of 3 single founders in their mid twenties that rent a two bed flat and then live and breath nothing but their startup collapsing on the couch each night can make two $50k angel cheques go a long way.
Edit: SEIS allows a friends and family seed round if up to £250k in London that the government will rebate 78% of in taxes (50% immediately and 28% if the company eventually goes bust).
I've lived in London for 8 years now as a student (undergaduate masters then PhD) and have lived on around £20k a year (post tax) and it's completely fine tbh. I earn more through extra work but save pretty much all of it.