Yes, it's true that the UK economy is very London-centric, but the original poster was talking about the UK as a whole vs the US as a whole. (The flip side of this is that the figures would look better if you compared London to a major US city.)

None of this changes the fact that US software engineering salaries are a poor comparison to use to illustrate wealth disparities between the US and other countries, as they are an outlier.

Regardless, Americans are not five times richer than Brits by any reasonable measure. The salaries in the comparison upthread are outliers. The exact figure obviously depends on which stat you look at, but Americans are around 50% richer by most measures.