You two are correct, but exit checks were done away with nearly 20 years ago. Why did they do this? Well the UK no longer applied currency export restrictions and they found that they served no practical purpose. The checks were costly and required significant resources to support.
So they reintroduce checks for 'safety and security reasons' which sounds very hollow to me and pass the costs along to commercial businesses.
As dc9-32 says this is more about being seen as doing something before an election than about actually addressing the problems ...