A family of 2, 3, 5, 10? How old are the kids? Do you want to buy a house, rent a house, rent a flat? Do you want to walk everywhere or use public transport or do you feel the need for a car? Are you used to eating home-cooking or take-away foods? Will you be working as well or a SAHM? Do you wear designer clothes and shoes?
Everyone's lifestyle is different. Me, I'd say £65,000 per year would suit me and my family of 4 just fine. We survive on less than half of that. We don't live in London, but we are just 45 minutes away by train and live in an area that is getting more and more popular with "commuters."