It's not so much that we're bad at learning, it's just that we've never had to bother! The last remnent of Britain's days as rulers of the Empire is the use of English as an international language - so everywhere English people go, other people speak our language, so we've never bothered to learn theirs. Walk into any restaurant in Europe, and the waiting staff will all posess a minimum grasp of English necessary for you to order and pay for a round of meals and drinks. If the language of the world was, say, Chinese, probably spoken by more of the world's population than any other, the English would have to drop the unspoken assumption that everyone will understand us when we open our mouths, and get down to learning to speak another language as soon as possible. Hey presto - another language would be introduced into nursery education, as it is in Europe, and it wouldn't be the revolutionary idea that has been mooted in the papers that French is taught to younger children.
English people may be no worse at learning foreign languages than any other nation - they just don't have to bother learing, so they don't!