FYI, I've moved this from the University of Brighton forum to the Accounting and Finance forum, which seems more relevant.
In answer to your question, "prestige" has whatever value you ascribe to it. Also, it would difficult for someone else the balance the four characteristics you mention (graduate prospects, social life, costs, reputation/prestige). Only you can know which factors are more/less important to you. All other people can tell you is how important they are to them.
My personal opinion (and other will have different opinions) is that Warwick beats Manchester but not LSE in terms or prestige/reputation, and would likely have significantly lowers costs then LSE. Social life tends to be what you make of it, and is more influenced by your peer group that the actual setting. Having said that, Warwick university is in the middle of nowhere, so is likely to suffer from a lack of easy access to a varied social scene. The social scene in Manchester is likely the best of the three.
Are your offers from each uni the same? Or do you need to factor that into your decision-making too?