How were the sea levels around western Europe during the Medieval Warm Period?
In the global sense the sea level data are not entirely consistent, but then, there are always going to be apparent variations when you are comparing proxy data from different sources. As far as Europe is concerned sea level rose between 1.0 and 1.6 metres throughout the Medieval Warm Period - or maybe as much as 2.0 metres by some of the more extreme estimates. Following this was the 'little ice age' (actually not an ice age at all), when sea level fell by almost as much. See for example the graphs of Behre 2004, and Puls, 2008, (German North Sea coast),both of which are reproduced in Kwaad's paper, which is a good overview.

http://www.kwaad.net/SeaLevel-MiddleAges-LittleIceAge.html 

Whatever sea level change you regard as definitive, the important point is that these were just minor variations superimposed upon a major sea level trend. Overall the post-glacial sea level, from about 8000 years ago, was a rapid rise from -50 metres (or so), flattening off during the last two millennia, and now massively accelerating again, primarily due to human-induced climate change.