That's pretty average for dev jobs in commercial enterprises in London (e.g. working for a B2C). In the UK, the big money for developers is in finance (either at a bank, or prop trading, or insurance, or gambling, etc). If you want a shock have a look at the wages in some of the surrounding areas (e.g. Buckinghamshire, Oxfordshire) - you'll be lucky to break 40k£, even if your skill set is very specific to what is required. Contrasted with finance in London, you would be looking at 70-80k£ for a mid/senior, and 100k£+ for a true senior/lead.

Also important to realise that 50k£ is 2x the national average wage. So for more than half of the population it's nothing to scoff at.

The UK is also in a bit of a funny place when it comes to immigrating to the US - due to a lot of American businesses having satillite offices here and a lack of specific visas for Britons, you're going to have a hard time finding a company that will be willing to sponser and relocate you.

Another really important factor when looking at the UK labour market, specifically in London (because my experience with it is there, but this could be true for the other major UK cities), is that contract roles are much much much more lucrative that FTE (or at least, they have been prior to IR35). If you're a mid/senior contracting at a bulge bracket bank you could expect a ~500£ day rate. 500£ * 282 (working days - some holidays/sick/national holidays) = 141000£. If you run that into your contracting LLC, rather than take it through PAYE, you can cut your effective tax rate by a big margin depending on how you spend your money - so even more cash that you would be FTE on the same wage.