Presumably other European states being happy to own UK utility companies means there's money to be made in doing it, otherwise why bother. I don't see how it's preferable to have that profit leave the country than being returned to government coffers or being used to keep prices charged to consumers lower.
Nationalise all profitable companies then?
The issue of nationalised industries being inefficient due to the knowledge of a safety net of extra government support is a totally separate one that can be tackled by having competent people running them. It also didn't seem to stop National Express just handing back the East Coast franchise instead of it dragging down the rest of their business to keep to the terms of their agreement.
The East Coast franchise is a famous example, but that was due to poorly managed process and note I haven't actually been critical of plans to nationalise rail. I don't have a concrete position on that yet.
Experience has shown us in the long term, nationalised utility providers have been poor value for money.