The cost of rent, food, drink, everything's quite expensive as it all has a London markup.
Say if you earned £20,000 gross but you pay £90-100 pw in rent (about usual for a reasonable place) it's like grossing $40,000NZ but paying $220-250 per week in rent for a place in Wellie. So you don't have as much disposable income.
Plus there's travel, (around £20-30 per week usually, unless you live close to work which if you want a decent job isn't likely as they're all in tube zone one and you'll probably be in tube zone two/three), the fact that everything's more expensive in London - e.g. for a pint of beer you'll pay close to £3 in most places in London.
But get out of London - say Birmingham, where I am - you pay £2 for a pint, food's a bit cheaper, rent DEFINITELY.
And converse is you're joining the pound-a-pill generation over here - in other words if you don't drink, you can get away with spending about £25 on a decent night out, including entry ticket, about three, and a bottle of water. Not that I take those sorts of things, I just heard about that.