Ok. i will give an example for income, savings and costs,,
an average java developer with experience (say 3+ years) will earn around 35k per annum or 40k per annum,, but in investment banks or in finance domain it can be 50k
(all figures in pounds and per month)
income (monthly,, net of tax , say for a salary of 35k) = 2200 pounds
rent(say 1 bedroom flat/apartment) (assuming the flat is in zone 2 or 3,, i.e about 30 min max to central london) = 800
council tax = 100 (max)
monthly travel card (zone 1 to 2) = 90
food (mostly preparing at home,, and some occasional eating out or ready to eat stuf ) = 200 (max)
car insurance = 30 (max)
bills ()water, electricy, gas, internet, phone) = 150 (max)
gym = 40 (max)
mobile phone bill = 40 (max)
other (just an allowance,, for night outs etc etc) = 50 (max)
savings = 2200 - (800 + 100 + 90 + 200+ 30 +150 + 40 + 40 + 50) = 700 per month.
This is accoording to my experience. note that figures can vary,, but i think this reflects somewhat a true picture.. note that expenses are up to you to control,, of course you can do night lifes or clubbing/ pubs everyday and save nothing

,, also note that 35k pa is average sal,,
hope this info helps,,also note that the figues are for a single person..
if a coupls comes,, then you can defintely save more, since everything will be shared..of course assuming the partner works and chips in with some income

,, the same 1 bedroom flat will suffice,, so will save on that..