A further observation... the key element in English music (southern English, at least, which I believe is what you want to play) is rhythm. Irish music focusses more on melody, and the decorations serve to embellish the melody.

The rhythm in English music is often more subtle than might first appear, slightly off the beat and sometimes with internal rythyms that are easily overlooked if you don't listen carefully. Compare the way Oscar Woods played his eponymous jig with the way most session players do if you want an example of this. If you can get the feel of the rythm and play accordingly, I believe this will help you to fit in far more than the question of decoration, or lack of it.

It's about the "feel" of the music - if the style you're playing "feels" Irish, you won't fit into the session, no matter what historic justification you can produce for the decorations. If you can give your playing an "English feel", by understanding the English rythms, then I suspect any decoration you choose to add will be less of an issue with the other musicians in the session.

The styles of playing most instruments has moved on from the original tradition. Many traditional melodeon players seemed to use the basses more for percussion than harmony, whereas modern players feel free to introduce more sophisticated harmonies. Anglo concertina has likewise developed a "revival" style. Many modern fiddlers, including some of those mentioned by Johnny, seem to include more decoration than the "old boys" did. But the music still has an English feel to it because of the rhythm.

If you can get the rythm right, the rest will follow