I expect that the Pension Funds wouldn't mind that much because the proposal is for the comrades to decide the value of energy companies and compensate their owners in gilts, i.e. by printing money! That is why the idiots think it will cost nothing!
Of the big 5 energy companies all but one are owned by EU energy companies with some of their shares owned by their respective states. Their owners will not see UK gilts as a suitable substitutes in their investment portfolios.
In my view there is no evidence that the Labour Shadow Cabinet have thought this through whilst considering in any way the effect on the public. What foreigners will see is the UK government failing the follow the rule of law and the effect on overseas investor confidence will be catastrophic. That loss of confidence is why McDonnell expects a run on the pound and is right to do so. Expect a far bigger loss of jobs and GDP than Brexit could ever summon up.