I lived in a London 10 years ago, and both salary and expenses were pretty similar. There are a lot of cheap or free (public museums and art galleries are free) things you can do in London if you know where to look.

Unlike SF travel is a heck of a lot cheaper, because there is usable public transport. I lived in zone 3 and worked in zone 1 and typically paid no more than £20/week for travel (usually by train or tube) - compare this to SF where a single Uber trip across town could be that.

I was there again last year for a few months as a contractor, and prices hadn't changed that much other than housing:

This time I rented a studio in zone 2 which was £1200/mo. It included water and heating, and I paid around £20/mo for electricity (with a pre-pay meter so quite expensive). This was a short term contract though, for long term I saw similar places for £1000/mo or less.

I usually ate out for lunch, which was £5-£10/day. I would cook dinner, spending around £20/week on groceries. Coffee was around £2.50 for a cappucino. Sometimes I walked to work, getting breakfast out on the way which was another £2 or so.

I took the bus, cycled or walked to work, so my travel expenses were under £15/week. I'd go home to my family at weekends, so can't comment on 'going out' costs.

Compared to before it isn't that much different. You can live quite well as a single person on £1500/mo in London. That's leaving nothing on the table and is above the living wage, so you can see where the problems come from.