Well by biggest resentment would be the Norman conquest. We have never actually fully recovered in restoring our ancient traditions. Protestantism of 17th century tapped into it, but went down another path.
I believe that there were errors within Medieval Europe, that Popes abused their positions. That Protestantism was stimulated by certain things such as nationalist renewal and that Catholic Church antagonised some of these notions.
There was some Celtic influence yes, but was predominantly Roman. Don't forget England was a heptarchy and religions conversions usually began with the kings themselves being converted.
The French are pretty filthy. The Anglo may have varies political issues (I.e unhealthy tradition of democracy), but I have to say at least we didn't engage in race mixing like the Continental nations when colonising.
