Lamprey 'eel', like salmon, has indeed been a dish for the privileged - nobility and royalty [and poachers] in Britain and Europe, but Lampreys also have the rather dubious privilege of a reputation as a means to cover the presence of poisons.  Most older versions of King Henry 1's demise imply that there was a little more to it that mere "over indulgence".   There are several traditional folk songs of some antiquity from the British Isles that refer to poisoning by 'eels' or lampreys "boiled in broth {sauce}", probably because the sauce was strongly flavored and rich enough to cover the 'off' taste of various poisons - and maybe the natural flavors of the lampreys and the sauce might have covered the taste of spoilage also?