Wharton definately has a better postgraduate business school than both of them, but as an undergraduate school, it might not be so great.
Wharton's (undergrad) main advantages are really its reputation and the employability of its graduates. The average salary for Wharton graduates was $55,000 in 2005, which is higher than the highest salaries in the UK (LSE's £25,000 just misses.) That being said specific schools of management or programs in finance / economics from top schools might beat that.
In terms of cost, you're best off staying in Britain - if you do not receive financial aid, Wharton can cost up to $200,000 for 4 years.
Also, Wharton forces students to take many 'redundant' courses as a part of the Liberal Arts Curriculum. 47% of courses are done outside of Wharton in the other schools at Penn. A degree takes 4 years to achieve, and even after that, my personal opinion is that you get a more indepth look at your subject if you stay in Britain (not to mention you graduate in 3 years.)
Overall, I think Wharton has a definite advantage in job placement. Whether this is worth the $150,000 more that you end up paying for it, I don't know. Warwick and King's are both great universities that will offer you plenty of job opportunities/opportunities for graduate study. Wharton has a great reputation, but internationally being a student at a top school in Britain is almost as good.