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.
Comparing salaries internationally based on currency conversion is never meaningful because it ignores differences in cost of living, tax rates and so on. I have always found it more productive to compare salaries based on ratio to average full time earnings when I am considering moving countries. That is, I assume the local economy is such that someone who is working full time can afford decent housing and lifestyle. This is not always true (London!) but is reasonable because a city would usually collapse if people couldn't live there with a job.
Using the tried and true method of a quick google search, I couldn't find individual earnings, but US average household income was 41 355 USD and UK was 27 029 USD in the same unstated year OECD report. Based on your estimate of UK salary at 60k USD, that is about 2.2 times average household income, which is actually higher than the 1.9 ratio for your figure for US.
Let me give you some first hand insights from someone who did her PostDoc and a PhD in the US respectively Europe and then got hired as a computer science Lecturer three years ago in the London area.
Short Answer: The academic salaries in the UK are in no way competitive to salaries in the US even when considering only the 9 months US salaries. In some disciplines (e.g. computer science) a US PostDoc salary enables a better lifestyle than a UK Lecturer salary.
The Long Version
Salary scales are evil. In my opinion, the main reason for the low UK salaries is the "salary scale" system, which is the same for all public universities. Yep, that's right: It won't matter if you have snatched an offer from Imperial/Oxford/Cambridge or, for example, from a much lesser ranked institutions such as the London Metropolitan University, you will roughly earn between 38k-45k GBP a year as a starting Lecturer.
It also does not matter if you are based in a less popular area where you can rent a two bedroom house for as little as 600 GBP a month or in central London where you will have troubles finding a room in a shared flat for the same price. (I am not exaggerating, feel free to check the data. To be fair, the London universities offer a "London allowance" in addition to the base salary which adds 2k-3k per year amounting to maybe around 100 GBP extra per month after deductions - note that these allowances were fixed back in the 90s and are rarely increased ever since.)
Why am I saying that salary scales are evil? The problem is that some of the more reputable institutions would be able (and maybe even willing) to offer their staff higher compensation. But having salary scales essentially requires the scale adjustments to match the amount that is affordable for even the least endowed institution.
In my case, taking up the Lecturer (Uk) job after doing a PostDoc (US) means that I'm struggling to even afford the lifestyle that I had as a PostDoc. Compared to a US assistant professor it feels that I am earning 50% less. Again, this is not an exaggeration and a simply calculation by checking the UK academic salary data and factoring in the cost of living. [My university is frequently ranked among the top ones for computer science in the UK thus for others this might be even worse.]
The projection of "closing the gap between UK and US salaries" due to the union's salary negotiations as outlined in the accepted answer by @StrongBad is wishful thinking: Last year's increase was 1% and this year the so called improved offer amounts to 1.1%. Strike actions by the union had no effect whatsoever.
Recommendation:
Stay clear of the UK academic market especially when looking for an entry level position. The situation is slightly better for higher ranked positions such as Reader or (Full) Professor.