At 1/12
You give me £12
If Labour wins, I will give you your stake back.
I will also give you £1
If Labour doesn't win, I keep your £12.
So basically, would you stake £12 just to win £1?
At such low returns, they basically think Labour is bound to win and aren't offering you any incentive to try and win money - because if everyone put money on Labour to win, the bookmaker would lose.
(But I bet someone's probably explained that to you)