The soldier said his snub came a year after he attended a Downing Street reception during which he claimed the PM appeared "totally disinterested" in him and other servicemen.
So on the basis of his opinion that he "appeared" uninterested he then gets involved in a pretty petty insult.
Those claiming its ok because they dislike the party should perhaps ask the question of when someone uses the actions of said Lance Corporal to act inappropriately with a senior officer or HM herself then will you think person is acting rightly or wrongly.
If he wishes to protest then do so out of Uniform not in Uniform.