I can assure you that A level choice is important for any uni that'll give you a realistic chance of being an investment banker- you'll definately need maths and possibly economics. For example, Leeds Uni prefers applicants to have at least a B in A level maths for Economics, the same with Business and Financial economics.
As for saying that Oxbridge people are the only ones in the industry, the majority of banks recruit from the top 10 unis in the country (being completely realistic, I don't like your chances of doing investment banking with a degree from Leeds, let alone Leeds Met), with a preference towards the better Unis such as Oxbridge, LSE etc. Of course there are exceptions to this rule, but these people are in the minority.
Also, saying that the 'social life at oxbridge smells of arse, gotta enjoy the 3 years' is a little irrelavent. If you want to do investment banking you want to be looking at maximising your chances- i.e. summer/easter internships throughout uni etc, as well as a good class of degree. It's not something that a graduate who concentrated on the social aspects of uni will find much success in, especially when they realise that the job involves extremely long hours etc.