The most important is Gisela Stuart, though honourable mentions should go to both Kate Hoey amnd Ian Davidson. Gisela Stuart is one person who really does know her onions on the the whole Constitution as she was one of the sixteen members of the Constitutional Convention's Preasidium that drew up the Constitution back in 2004.
Her horror about how this was done came to light when she published a paper through the Fabian Society about how dictatorial the process was.
"The Convention brought together a self-selected group of the European political
elite, many of whom have their eyes on a career at a European level, which is
dependent on more and more integration and who see national governments and
national parliaments as an obstacle. Not once in the sixteen months I spent on
the Convention did representatives question whether deeper integration is what
the people of Europe want, whether it serves their best interests or whether it
provides the best basis for a sustainable structure for an expanding Union. The
debates focused solely on where we could do more at European Union level. None
of the existing policies were questioned."