It's a pun that slyly points us to a metaphorical interpretation.
The pun lies in the two senses of the word power: electrical current and political clout. The metaphor consists of the insistence on electrical power being interpreted as a symbol of the political power. There is a literal component and there is a figurative component; the former is unarguable fact, the latter is a figurative interpretation.
Taken together, the association is drawn to get a laugh. The "Metaphor alert" is used to force the association, because the author wants people to see the humor in it. Those who support the PM will probably not find it humorous, but her opponents will. That's politics.
Let's take a simpler example of metaphorical usage: say the Queen were to die and the PM were to say "A great light has gone out." That would be a pure metaphor, with no pun intended or expressed (and none intended here).
But now let's say the Queen were to die because she accidentally stuck her finger in a light socket and that tripped the circuit breakers in Buckingham Palace, plunging it into darkness. Now the statement "A great light has gone out" would, perhaps unintentionally and certainly unfortunately, achieve a humorous effect, because people would likely see the association without prompting, and likely be offended by it.