By undergraduate student quality:
1. Cambridge 218 points
2. St Andrews 215 points
3. Oxford 209 points
4. Strathclyde 203 points
5. LSE 202 points
6. UCL 198 points
7. Glasgow 196 points
8. Edinburgh 188 points
9=. Durham 186 points
9=. Warwick 186 points
11. Bath 180 points
12=. Aberdeen 176 points
12=. King's College London 176 points@9572 - Cambridge has better quality undergrads than Oxford. And Imperial doesn't even offer Economics at undergrad. Seems like you're very poorly informed.
The averages aren’t even particularly relevant since the gap between Oxbridge and others in undergrad quality is high (and Oxbridge’s interviews ensure that very talented students whose marks may be slightly lower are admitted).
The top students only really can have any serious impact on the profession and LSE, owing to its history in the field of economics, its high quality faculty and location, enrolls some students from the far right tail as well. The remaining top economics students enroll elsewhere and spread across a number of places (though average high school students may find a passion for intensively studying economics at the university).