Well, there are many factors to consider really but the word 'career prospects' is not right. No matter where you work, once you have local domain experience you will grow. As long as its a developed country or a country on the fast track in a field like India in IT, you will always have a career and be as successful as you want to be
The next thing to consider is work life balance and salaries one can get. It is well known in the UK that a majority of the companies allow you to have a personal life. Mostly work is 9-5 unless at times where its required to stay back to sort issues or a late night project go live which is same all over the world be it UK or the US. Importance is given to personal time, employees with kids have many benefits and options of flexible working. There is a lot of respect at the work place which UK is very well known. You dont need to put with any constant long hours without extra pay or bullying at work etc....
Salary packages are really down to the individual. If you are in a domain that has demand and a well experienced guy with say over 5 years min, then you can look at jobs paying in the region of £50k to £80k. More senior the role, bigger the company and even better the pay. I have friends who are 32 and earning £90k plus bonus and allowances which is approx $153k - but Im not sure if this is good or great in the US. Companies like E&Y, Deloitte, Accenture, IBM and so many smaller outfits pay a lot - and this is just IT. Financial institutions pay a lot more and doctors here live in luxury
You have to note that very similar to the US, getting these jobs need that extra skill and once again all this is the same be it US or UK.
Your life choices, what you like to do outside of work, the weather, facilities and benefits the country gives, visa issues and long term goals all play a part in deciding if you want to move to the UK or not - career progression in itself is similar in the UK and US as both countries are a hub for the best companies, there is global exposure and there are opportunities to make a name for yourself
KP