Firstly, you're talking about a failure of a regulator to regulate an industry and prosecute alleged perpetrators, and the implied solution is to… nationalise the industry, somehow assuming the nationalised entity will be managed better by the very same regulator? I don't think it's a good argument really
Secondly, if we go even deeper, what pressures would such an entity have to provide good service and not be wasteful with their resources? Right now there's at least market pressure, the companies trying to outcompete their peers, cut costs, and ensure future profitability by investment. What pressures do you think exist for a nationalised entity that don't exist for a regulator?
Thirdly, do you actually have any evidence to prove that those companies are uniquely profitable? Because unless they are, it's only fair that people investing their own money into infrastructure (remember, that was one of the goals of privatisation, tapping private budgets in addition to tax money) get some return on it, otherwise they'd just invest it elsewhere, e.g. into Apple stock, which would benefit British people much less.
Privatisation per se is not the problem, political failures are.