Food and drinks are usually quite cheap on campus in the cafe or restaurants which are often of a decent portion per person. Even locally there are usually good deals for students on within the restaurants itself. At Guys campus, it's very easy to just go to the high street to find some food or if you want to go upmarket, Borough market is just across the road and if you take another 5 minutes walk there is the river where more restaurants are located and on the other side of London Bridge station on Tooley Street. On the other hand, Central London is only a short bus/tube ride away so you're never stuck for food!
Part-time work is advertised sometimes through emails, although with the number of retailers in London you can easily find some form of part-time work!
You should allow for some money for books to be bought and usually there are students who are willing to sell you their old books for a much cheaper price. Transport in London is discounted for those who have student oyster cards and it is cheaper to attach a 16-25 railcard onto it for those pay as you go journeys.
Just make sure you budget well, give yourself a limit as to what you spend and you should be fine at a London university. There are plenty of discounted/cheap activities for students but you just need to have your wits about you! But remember that you are also in London so things are going to be more expensive e.g. rent.