The problem is the single person making £15k. They get to keep £12k so they have £1000 left after taxes. A studio flat in London will cost at least £500/mo. Add council tax, utilities, a monthly travelcard for at least £100/mo, food and clothing, and things start to get a little tight.

A couple sharing a 1 bedroom flat with two incomes are clearly much better off.

"It's the statue, man, The Statue."