I'll declare it up front, this is an idea totally stolen from Stella Creasy.
Water companies failing to deal with leaks, energy companies over charging etc. Regulators like Ofgem and Ofwat quite regularly fine big companies running our public utilities when the mess up. We could use the money raised from these fines to buy shares in the companies which can then be given to customers and employees alike to mutualise the service.
Mutualisation has benefits over pure nationalisation in that the co-operative owners have a greater, democratic say in how the organisation is run but a mutual set-up also makes it much, much more difficult for the Tories to re-privatise them when they get back into office.
Look at what the Tories are doing in office - they are seizing the opportunity to radically alter the state in ways that are practically or politically difficult to reverse (boundary changes, trade union restrictions, privatisations, English Votes for English Laws etc.). We need to get much smarter at locking in the social gains our years in office allow us to enact so that our good work can't be un-done by a few years of Tory rule. This policy is one example of how we can reassert our values of public ownership whilst locking it in for the long-term (and making it more democratic too!).