Look at Mr Red's Festivals
Oxford FF is not that far from London but is 16th -18th and big on names.
Priest Weston (Shrops) is always Easter, and Miskin (nr Cardiff) is usually Easter, try the links/phone num on my site for better info. More of your session/singist events.

We have a modest Ceilidh at Randwick VH on the Easter Monday in Stroud. See dance dates.