Between approximately 60,000 and 100,000 years ago there was a land bridge between mainland Europe and the United Kingdom. Now, this is covered by the North Sea. Dutch trwalers, dredging for sole, scrape up amongst their catch a bonus haul of sub-fossil bone, including mammoth, woolly rhino, bison, and giant elk.
These bones are from the mammoth Mammuthus primigenius.
The bones are priced of a few factors - size and damage, for example - so size varies. They'll most likely be rib sections roughly 15-20cm long.