That plan is neither comprehensive nor fully costed. They would be in breach of international law because they would be taxing companies on profits generated elsewhere. That's unsustainable, because effectively every country could levy tax on multinationals to the point where they tax bill is greater than the company's value.
e.g. if the UK charge Shell 20% tax on profits, and so did the US, Canada, France, Germany, Italy, Japan, China, Australia it would quickly become impossible for them to pay their tax bill.
Also why shouldn't prepay meter customers pay more? It's more expensive to supply energy that way, more infrastructure is needed.