On the main land there are many archeological findings of bows and arrows, while in the U.K. there is the Mary Rose, and furthermore, euhrmmm, ...... nothing...
Of course the Mary Rose is a huge finding but only gives information of warbows and arrows at that particulair time.
Not strictly true. Don't forget the amazing find at Westminster Abbey. This arrow is far more complete than any found on the Mary Rose, it has a fully complete type 16 head and dates from a much earlier time than the sinking of the Mary Rose. It doesn't have a bulbous nock, but a 3/8" horn reinforced self nock. It's also a bit shorter than the MR arrows, either a sign that the MR arrows are a variation from the norm, or that the arrow was repaired numerous times and stopped being a war arrow to become a personal target arrow. The type 16 head certainly suggests it was still used for human enemies as compared to hunting however.
There are also the amazing finds at Portchester Castle, again dating back to the time of the Mary Rose but the arrow heads found there are complete, and are in fact what the EWBS Tudor Bodkins are based on.
Most of the painting that depict bulbous nocks are not representative of English archery, they're usually representative of whichever archery the artist was familiar in his country. For example the many paintings of St. Sebastian, which you see in just about every European museum, depict the archery of the country in which it was painted. The period of most of those paintings coincides with the domination of he eastern Mediterranean by the Ottoman Turks. The Ottomans did have such arrows, I've seen them in Museums throughout central Europe.
My guess, the artist saw such arrows and placed them in his paintings. Those bows were also shot on the same side as the bow instead of across.
I still can't get myself to believe this theory when people mention in. The artists who drew images of archers during the 100 years war would not have just been isolated scholars and illustrators hunched over candle light. It was a time where every man of a certain age was required to shoot a bow. Archery in those times was law, entertainment, lifestyle and leisure. It's highly unlikely that any man who lived around archers would have got things like that wrong. Especially so many of them. If one painting was found with stylized nocks, or the arrows on the wrong side then perhaps it could be assumed to be a mistake on the part of the artist, but with so many depicting this, in so many styles from so many years spread throughout the period, it's a hell of an assumption that they were ALL just making it up based on memory of something that was used in a different part of the world.
It's very hard of course to accurately state how these paintings came to be, but to begin with I think there are two basic possibilities.
1) The artist himself was present at a battle. This is slightly unlikely due to the risk of death or capture etc, but you never know. If the artist was present, there wouldn't be mistakes like getting the arrows wrong or the shooting style wrong. It would have been right in front of him, and why go to the extra effort of creating a nock that is so different to that which is being used in the battle?
2) The artist draws from people telling him what they saw. This is far more likely and even this doesn't have room for specific stylization of a weapon. Either the person recounting the battle would scribble bad drawings for the artist to use, or describe in full detail what they saw. To add a bulbous nock to a straight arrow is a big deal - it's far easier to draw a straight arrow, and that's what anybody who was present would have drawn, and certainly somebody recounting wouldn't have added a bulbous nock in the description for no reason.
This leads me personally to believe that bulbous nocks WERE used, and were definitely present. I can't see any plausible way for an artist commissioned to replicate as closely as possible the images of battle to make up and add such a specific, geographically-limited feature such as a bulbous nock on a tiny drawing of an arrow unless he was absolutely certain they would have been there.