if a firm has a regional office, its almost always better working there consdidering the pay, London is expensive, REALLY expensive, everyone is getting priced out you can 100% see why everyone wants a US firm...
FWIW, London is not expensive at all (I live in Zone 2) in that public transport is cheap, there are lots of cheap supermarkets etc, you can go on holiday to Europe (etc) for small sums of money, Universal Credit is actually still available for single people paying higher rate tax (£50k+): so long as you are renting and single it's going to be government-subsidised; however, in the long-term the property market
is deeply broken and it's millions of pounds to buy the sort of housing that the income percentile that city lawyers should be able to buy. At some point when you have equity, city law, etc. then clearly London makes sense, and if you are willing to occupy a very small amount of real estate, typically renting, it does give high take-home pay, but ultimately government failures mean that London is realistically a mirage for most people.