This is one of the 2012 league tables:
http://www.theguardian.com/higher-education-network/2012/jun/29/top-uk-universities-for-environmental-sciences-2012-qs-world-university-rankingsOne of the things is, though, that the higher-ranked universities (probably those that you'll have heard of - because they have a strong international reputation) will be those less likely to accept you into your 3rd year: they may take you in to the 2nd, but this would mean an extra year of living costs and fees (though as an international student, you'll be allowed to work 20 hours a week, I think - I remember being extremely bitter that, many years ago, the US did not permit international students to work: which meant although I have post-grad from three countries, none of those are the US!) - if you're really determined, though, it might be worth it.
You could check this out too -
http://www.whatuni.com/degrees/courses/degree-courses/environmental-science-degree-courses-united-kingdom/m/united+kingdom/crh/8563/page.htmlIn all honesty, I suspect you'll have better luck with post-92 institutions (those that were originally polytechnics) a) because they are more flexible in a lot of student-related matters and b) because they are usually less over-subscribed or have a higher drop-out rate (technically, we can only take students into later years of a degree if there is space on the course, though this isn't rigid at all universities) - from a quick look, the highest ranked of the courses from a new university is that offered by Gloucester. However, because it is quite a specific course you are interested, you might fine that anyway a new university offers modules more to your taste, because they tend to be more practical (though not always) in terms of the things they cover - more policy than theory, for example.
However, the good news is that you're an international student, so there will be more flexibility that if you were a home student: we love international students because you have to pay lovely high fees (British students' fees only covers part of the actual cost to educate them per year; international students cover it all) so we'll forgive you a lot ;-)
(Why do so many Americans study at St Andrews? Apart from anything else, it's blooming miles from the nearest airport :-D)