Where did the tradition of Longbow use in the British Isles originate?
I'm an avid practitioner of the Sling and while doing a bit of research on the weapon I noticed that very few if any early records/depictions from the Iron Age to the Roman Period mention the use of the Longbow among the Britons/Celts/Picts.
Curious I checked Wikipedia which gave some useful links and the familiar history I learnt in school detailing the weapons use during the Middle Ages, at Agincourt and Crecy and how bow training was encouraged and passed down etc etc. It also mentioned that the first Longbow was discovered in a bog in the Alps and is thought to have dated from 3000 B.C.
So I thought maybe the Saxons, Danes or Germanic Tribes brought the weapon over with them. I thought on back to High School history and how the Saxon Army that faced the Normans at Hastings was very light on archers. Williams army seems to have had archers but from the scattering of sources I found they look to have been smaller and less potent than the stereotypical Longbow; they also look to have been very much a secondary weapon.
I then found a few quotes from the so named Gerald of Wales who mentioned encountering the weapon in the 1100's in use by the Welsh however the way he mentions it was as if he had never encountered one (indicating that the weapon was not common in England) not long after 1066. Fast forward a few hundred years the Longbow is being used by Yeomen and Commoners across England, forming the backbone of the English Armies during the Hundred Years War and with its use widespread and ubiquitous.
TL:DR: How did go from Ancient Britons with Slings and Chariots plus the odd short bow to the Bow-lite Saxons and Normans to the Bow heavy Armies of Edward III (and the ubiquitous use of the weapon in the British Isles)?
It also mentioned that the first Longbow was discovered in a bog in the Alps and is thought to have dated from 3000 B.C.
That's weird because the oldest longbow found in Scotland (in a peat bog) has been carbon dated at ~4000 BC.
however the way he mentions it was as if he had never encountered one (indicating that the weapon was not common in England) not long after 1066.
That's not a good interpretation. He mentions that the Welsh were using elm bows and specifically notes that their bows weren't of horn or yew.
"The bows they use are not made of horn, nor of sapwood, nor yet of yew. The Welsh carve their bows out of the dwarf elm-trees in the forest. They are nothing much to look at, not even rubbed smooth, but left in a rough and unpolished state."
That sounds like someone who knows a thing or two about bows and their construction to me.
they look to have been smaller and less potent than the stereotypical Longbow... ...the odd short bow
These too are myths. There was no 'short bow' before the longbow came about. The longbow is called such not because it was longer than previous bows but in contrast to the crossbow. Bows have always been long, the previously mentioned 6000 year old Rottenbottom bow was 1.74m, Scandinavian bows like the Vimose/Nydam (~5th century) examples are a little longer still 1.78m and a 10th century Irish bow was 1.85m. The 'shortbow' was never a thing.
So there are a couple people around here that might be able to answer the question better than I can, but I will do my best.
While /u/Muleo is absolutely correct in that the longbow itself originates far before the period of its largest usage, and that there was no real "short bow" in existence, I think perhaps your question can be somewhat paraphrased as "Why did English armies begin using longbows en masse and where did this tradition originate?"
The best information we have on the subject comes from Both Gerald of Wales, who you've already cited, who indeed states that the Welsh (Southern Welsh actually, which is an important distinction) were well known for their use of the bow, particularly in (relatively) close combat with the enemy. He states that the Welsh would charge forward, get within close range and unleash their arrows, allowing them high penetrative power, before retreating back. This was indicative of the kind of warfare the Welsh were fighting at the time - mostly against armored cavalry retinues of the Norman Marcher Lords.
The next best source we have comes from J. E. Morris' The Welsh Wars of Edward I. While a quite dated source, Morris is still being cited by modern day scholars of Edward I, notably Michael Prestwich. This is because Morris was peerless when it came to his research and understanding of the Pipe Rolls, the literal rolls of parchment that show the payment and size and makeup of Edward's Armies in Wales in the late 1200s.
Morris comes to the conclusion that Southern Wales, which had been more or less subsumed in the English sphere of influence by the time of the Welsh Wars, provided significant amounts of troops to the English armies, particularly those fighting in central and southern Wales. In addition, they were noted for their success at the Orewin Bridge, which actually led to the death of the last native Prince of Wales, Llywelyn the Last.
Anyway the TL;DR is that Edward took full advantage of his new Welsh dominions during his wars in Scotland, including it seems a hefty portion of Welsh archers. Whether they had developed what we would now term the 'Longbow" by this time is debatable as there's a dearth of sources recognizing it as such, but what is certain is that there were sizeable numbers of Welsh at the battle of Falkirk against William Wallace, and that they played a crucial role in breaking up the Scottish pike formations in order to allow the English armored cavalry to charge home.
After Falkirk archers start to play an ever increasing role in English armies - at Duplin Moor a small army with a large force of archers defeated a much superior scottish force, for instance. By the time of Edward III, Edward I's grandson, archers were a sizeable portion of English armies, and would go on to cut a swathe through the French nobility in the Hundred Years War. it should be noted that many of the archers who served in France also seem to have been Welsh, though with a growing English contingent.
So the conclusion is that what was most likely a South Welsh combat technique evolved during the period of Edward's Welsh Wars to become an important instrument during Edward's later confrontations in Scotland. Their success there led to their immense gain in popularity throughout the 1300s, where they eventually became one of the dominant features on Western European battlefields.
Citations:
Davies, R. R. The Age of Conquest: Wales, 1063-1415. Oxford: Oxford UP, 1992.
Morris, J. E. The Welsh Wars of Edward I: A Contribution to Mediaeval History, Based on Original Documents. London: Oxford UP, 1968.
Prestwich, Michael. Edward I. Berkeley: U of California, 1988.