I've lived and worked in the UK most of my life and moved to SF three years ago. There were a few things you omitted in your comparison but probably won't immediately apparent in the short time you were here.

Salary difference. On average Most decent experienced devs in London make around 50-60k pounds which is around 80-85k dollars. An entry level dev in SF makes anywhere between 90 - 120k dollars and most experienced devs earn at least 120k dollars with the range around 110-150k. So roughly a 30-60k+ dollar/year difference.

Healthcare. Almost all the companies I've worked for have covered my healthcare costs so I only have to pay copays - as an idea 20 dollars to see my GP and 30 to see a specialist. I had 8 weeks of physio last year and my total out of pocket was around 600 dollars including a CT scan, which I could pay using pre tax dollars. So the overall cost was pretty low. I also got to see a physio the same day rather than waiting 6 weeks+ on the NHS. My total max out of pocket is 3000 meaning I don't pay anything more than that. Given the significantly higher salary even if I were to hit my max it would still be worth it.

The cost of living is comparable although certain things are cheaper - gas, trainers(sneakers), car insurance to name a few. Food is roughly the same I reckon.

Overall working in London vs working in the US I saved significantly more money - it's not even close. Although a tin of heinz beans costs two dollars so that's made a pretty big dent in my savings ;-)