Are UK faculty salaries in computer science low compared to salaries in United States? I'm looking at faculty ads for computer science in the UK and noticed that the average salary range seems to be significantly lower compared to the US. Typically, a starting Lecturer ( = Assistent Professor) can expect an annual salary in the range of only 35k - 42k GPD which is in the range of 55k - 66k USD. 

On the other hand, if I look at the academic job market in the US, then this would be considered an average PostDoc salary at best, whereas the lower limit for computer science assistant professors seems to be around 80k. Considering that faculty gets this amount for 9 months in the US rather than 12 as in the UK and taking into account the higher income tax rates of the UK, this gap becomes significantly big. 

Does the UK system include certain perks that the US system does not have that I'm missing here? 

Are there other ways that UK faculty can make up for this quite significant gap?
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.