Okay, so I can't answer your question perfectly, because the study of time involved for peel decomposition is not very a pointed topic in the literature. Although, decomposition analysis of food and waste management are certainly topics that have received attention at a more general level...
A scientific paper from 2018 in Journal of Physics notes the constituent (carbohydrate) components of banana peel waste1:
the banana peel waste contains more fine fibers than those that are made of wood with high cellulose (60-65%), hemicellulose (6-8%) and lignin (5-10%)
It's important to note the hierarchy of decomposition for macromolecules from easiest to longest:
- quick – sugars and amino acids
- medium – (hemi)cellulose
- long – lignin; cutin
So already it looks on the surface that decomposers have difficulty dealing with various peels and cores compared to simpler targets. Of course, there may be specialized decomposers that, when present and given optimal environmental conditions, could certainly succeed. After all, detritus such as dead tree matter, is largely composed of cellulose; wood is largely lignin, requiring specialized decomposing rotters to metabolize tree stumps, for instance.2
Next, the surroundings and favorable situations for decomposition. You mention Keep Scotland Beautiful; however, you may not be aware that the estimated breakdown time provided is under the assumption of a fully aquatic setting. Aquatic and highly dry environments are not as conducive to some taphonomic processes, compared to areas less extreme. It follows that under a reasonable environmental setting, on land but where humidity lives, in which aerobic decay can be performed, it's possible that peels could degrade between the time required for leaves and foliage (~ 1 year), or possibly shorter, and the oft-quoted 2-year time horizon, but likely not less than the time needed for the fruit itself to rot in full with no remains or skeletonized to the least-degradable portions.
However, unlike the case with wood, the majority of rot in peels do not come initially from termites and fires in forest and grassland settings and other terrestrial biomes. In this sense, peels may not get a similar "head start".
Landfill settings and composting stations are completely different from the natural element. Landfills may be sterilized in ways that make it improbable for decomposers to "do their thing", or simply too voluminous for these to be carried in a reasonable time frame. For instance, studies conducted by University of Arizona researchers begot a book, Rubbish! The Archaeology of Garbage, the brainchild of William Rathje, in what became known as the Tucson Garbage Project, or colloquially "garbology". Primarily intersecting between recent archaeology and consumer spending and disposal habits, the findings also encompassed other practical topics of note. Of interest, newspapers and other paper, which represented 40% of all landfill (by mass, I think?), at least then, is a poor decomposer in these designations:3
"[E]ven after several decades, they remain intact and perfectly legible. The problem, the authors say, is that, landfills are not vast composters; rather, they are vast mummifiers. There is biodegradation, but its pace is measured in centuries, not decades. Even organic materials, such as food scraps, remain unchanged after 30 or 40 years. Mr. Rathje and Mr. Murphy cite an account of an excavation of an ancient Roman garbage dump in which the smell of putrefaction remained unbearable even after 2,000 years."
They also found out that people erroneously underreport the degree of their unhealthy lifestyle and alcohol consumption habits, but that's another topic for further reading.
What ideally would be a final ground for decomposition turns into a mummification temple, sans the brains being pulled out of people's eye sockets. Those involved with the project systematically analyzed 14 tons of waste, and the results were equally mind-blowing and consistent. This completely shattered people's long-standing belief that paper, including newspaper which has accounted for approx 1/3 of papers disposed, was a good decomposer and readily decomposes in landfills. Wrong. Radiocarbon dating is not required for this archaeology; all you need to do is identify the date of an adjacent NYT paper in the strata to know how long that half-eaten Manwich has been lying around there. Could be 35 years. I imagine the same can be said about banana peels.