Definitions of adjectival “only” from recent dictionaries:
“alone of its or their kind; single or solitary” (Compact Oxford English Dictionary for Students)
“used to say that there is one person, thing or group in a particular situation and no others” (Longman Dictionary of Contemporary English)
“single in number; without others of the kind; without others worth considering” (The Chambers Dictionary)
What all these definitions stress is the singularity of only, whether referring to one item or a singular group. If something is already singular, you cannot then have “one of” it.
Neal gives the sentence The 100 people in this organization are the only ones who know. This is covered by the Longman definition quoted above (“one group in a particular situation and no others”) .
Maxwell suggests we replace only by solitary in Waitrose’s statement, but for me that doesn’t make any more sense. He also says that if we remove the word only, the sentence is perfect. Of course it is: it is the juxtaposition of only and one of the that I cannot accept. Make it “Waitrose is one of only five IPs …” and I have no problem, or even “… one of only a few …”, “… one of the few …”.
In Waitrose’s statement, internet providers is qualified by only, but what does only mean in this position? If it means ‘few’, then they should have written ‘few’. If it means what the dictionaries all seem to say it means, then it is being misused.
Maxwell also suggests that I may be denying the existence of idiom. Of course not, but idiom also needs to be fitted to the style or register of the language being used. In a fairly formal style like the one that Waitrose is employing, care needs to be taken that the target audience will not be able to find fault with it. Inappropriate use of language often leads to a dismissive response by the readers or listeners.
Plus is defined in the dictionaries quoted above as an “informal” conjunction, but all the examples given show it as the first word of a sentence, not preceded by a comma.