I don't know about the UK, but this makes sense to me.
Working retail here in Canada, common rule is to prevent shoplifting through having eyes on suspicious people. However, you never go after them yourself when they leave the store, or even do so much as accuse them of stealing while they're in the store. You make security, who're specialised for the task and are workers with a contract company, aware and they recover the shoplifted merchandise.
It's for safety reasons and to avoid different lawsuits. The store doesn't want to be held legally liable for injuries or possible discrimination.