Someone who lives in Nairn recently actually travelled to Tweedbank and simply bought the ticket from his origin to his destination. There was no other appropriate through fare he could have bought for the route via Aviemore. So the ticket itself is clearly not a loophole...
You are splitting hair here, but yes in such circumstances it
can be argued that the fare is not a loophole, but simply a mistake. The passenger need not know the intricacies, as long as they are not denied their contractual rights to conveyance. I wonder how many people who bought these £1 fares fit that category.

(No, those who bought the ticket to do this trip to take advantage of the erroneous price do not fit into this category imo.)
....However if you say that the use of such ticket that is priced at a lower fare, and valid for a longer journey than you intend to make, ie starting & finishing short, is a "loophole" then the £1 tickets are just the tip of the iceberg (see below for another example).
Of course it is not the only case, otherwise a large section of the forum's traffic simply would not exist.
Because the fare seems too good to be true. Whether or not it's used as a loophole.
Agreed, but for a while a Hadrians Wall Day Ranger was cheaper than a Newcastle-Carlisle return. Obviously that was not intended, and when Northern increased the price for the latter they obviously forgot to increase the price for the former. So that's also a "loophole" as it's too cheap and was not intended to undercut the ticket with less validity.
Yup, can be argued to be a loophole, but also it should be remembered that the line between a loophole and otherwise can be a blurry one at times. Sometimes it is quite obvious, such as the erroneous £1 fare example (for the vast majority of purchasers, if not all, imo), but other times it is much less clear, like this example you gave as I believe the two fares serve completely different purposes since the return were an Anytime fare, so these fares were pretty much intended to be priced at their respective levels. You then have things in between where it is all differing shades of grey
Does this mean at times when the Ranger were valid it became a loophole? I don't know. I only gave examples of what in my opinion were obvious loopholes but even then, as the progression of this thread showed, some people vehemently disagree. What is the definition of a loophole? Maybe someone can try and come up with one but I suspect by its subject nature it would be virtually impossible, especially in fares and ticketing.
But, guess what? I asked for a Newcastle-Carlisle return at that time and was sold the cheaper Day Ranger (which I didn't even know was cheaper!). So ticket clerks will actually offer "loophole" tickets when you don't even ask them to.
I don't see how this is at all relevant to what is being discussed here.
The problem here is that "loophole" is subjective. Some people object to the term being used because it may have negative connotations.
I didn't mean for its usage to have negative connotations. That was very certainly not my intention. (I know this sounds crass but if some people have an issue with the word, then it is not really my problem because the word itself is a fairly neutral one.) If one can find a loophole and make good use of it, good on them. (This even applies to my attitude towards tax avoidance.) I am not in a position to pass judgement on people who do this.
If buying a cheaper ticket is a "loophole" then car drivers buying cheaper petrol is also a "loophole"
I disagree that it is at all comparable, as for a start you are not buying the same items, whatever your intention. Conveyance from A to B is conveyance from A to B (provided that they have the same, or at least similar, routing and operator restrictions). Fuel from one retailer may vary wildly from that from a competitor, or may be the same. I think we are straying off course here.
Also I didn't give a definition for "loophole", because I don't have one. I only know clear cases when I see one, well, clear cases imo anyway.
I wonder how many people would consider it a loophole if you randomly select 100 people from the High Street and show them the £1 fare example, explaining the whereabouts of the origin and the destination.