The marketing and advertising industry in London is pretty huge - so it's hard to give you a specific answer without a little more detail on the type of company you'd like to go into (PR, digital, social, experiential etc etc as well as in-house or industry). Finding work (if you have relevant experience) should be pretty straightforward. There are always tons of agencies recruiting - check the Campaign website for a pretty good list.
Again - hard to say what the expected salary would be without knowing more specifics, but if you've got three years experience in a PR or digital agency, you'd probably be at account manager level - so maybe 30-45k depending on the company you're working with and the specific discipline you're targeting. Quality of life on that salary is pretty decent. You'll likely be sharing a decent place in Clapham or somewhere similar with one or two others. You'll have enough money to go out/have drinks etc - maybe not every night of the week, but certainly often enough to want to join a gym...
Hard to say more, but you should make somewhere in the ballpark of 30-40k a year and have a pretty good lifestyle if you are economical and smart with your money.
