40K is around 2450 per month after tax. If you don't want to share a flat and don't want to spend more than 40 minutes commuting per trip allow around 1000 for a single bedroom flat in zone 2 or 3. Council tax is around 100/150 per month. Water/gas/electricity/internet should be between 150 and 200 per month. Monthly pass for zones 1 to 3 is 120 a month. So fixed costs are around 1500 a month. This leaves you 950 per month for food, clothes, gadgets going out, travelling and for saving some money. Healthcare is free for now.