The Times Online is reporting that Spanish is set to take German's position as the second most studied language in the U.K., right after French.
This change is in line with those languages' respective rankings by GDP. However, you might expect French to fall a bit, but being next-door neighbors likely keeps it up there.
I'd imagine that the popularity of a given foreign language in a given country is based on three things: the economic opportunities in the language (of which the global GDP represented by such speakers is probably a pretty good proxy), the country's proximity to speakers of that language, and the history of that language in the country.
With France, Spain, and Germany all among the wealthiest countries in the world and all nearby, plus a long history of strong relationships with all three countries, it's no surprise that these make the U.K.'s top three.
According to the National Centre for Languages' report, Chinese is also on the upswing in a major way. Whereas Spanish is in 50% more schools than in 2005, and Italian in about 150% more, Chinese is in 600% more. This would suggest that the perceived opportunities that Chinese has to offer are growing, and that's in line with their GDP ranking.
What I'm left wondering is where is Japanese in all this?
Related: The (roughly) top 20 languages by GDP