In that case Ethel everyone who operates an electric mower, strimmer, hedge trimmer is in breach of regulations?
I think the key point here is that phiz is running it from a socket (just like for garden power tools) and it therefore counts as a temporary installation. If phiz wanted to permanently wire the garden lights into the house electric circuits, then the regulations would apply.
With regard to the ELCB, you should use one between your socket and plug whenever you use garden power tools and would be well advised to do so in this instance as well.