Are UK faculty salaries in computer science low compared to salaries in United States?
You are trying to compare a US salary to a UK salary using the current exchange rate. I do not think this is particularly relevant. Even if you are willing to ignore differences between the UK and US in terms of work/life balance, teaching load, and job security, you probably want to compare the quality of life that a salary buys you and not what would happen if you converted it to USD. The starting salary for a CS lecturer/assistant professor, when converted to USD, is much lower for a UK academic than a US academic, but there are differences between the value of a US and UK salary.

While you say the UK tax rate is higher, that is a huge over simplification. Two random online tax calculators I used gave a US income tax of 24% on an 80,000 USD salary and a UK income tax of 25% on a 42,000 GBP salary. This ignores US state (and possibly city) income tax. The UK VAT of 20% is much higher than typical US sales taxes, but not everything is subject to VAT/sales tax. Even if you can accurately calculate an average total tax liability, you still need to account for purchasing power. 

I have work in both the US and UK in comparable cites (not NYC/London) and tend to think that the value of my UK salary is a little lower than value of my US salary. It is worth noting that UK salaries are much more consistent across fields. This means that I make the same as a CS lecturer in the UK, but that in the US I would probably be making 10% less than a CS assistant professor. My guess is that for CS there is a substantial, maybe 15%, hit in quality of life that an academic salary buys.

In the US your starting salary is often your salary until you get tenure/promotion while in the UK you get a larger salary every year. There is a performance based pay raise, that essentially everyone gets, which is generally about 2.5%. This means the gap between the US and UK values gets smaller every year and after 6 years, when one is preparing to be promoted, the gap is pretty small. Further, UK salaries also have a cost of living adjustment. For the past few years this has been about 0.5-2% and less than the inflation, but it wouldn't surprise me if the union negotiates a big salary bump, maybe 10%, in the next few years. This would really close the gap.

It is also worth noting that a US assistant professor is not directly equivalent to a UK lecturer. I think a US assistant professor is often a few years ahead in terms of productivity and experience.