Here's a link to the Warwick Folk Festival programme. I can recommend the Roebuck for (mostly) accompanied singing and a few tunes, and the late night sing in the Arts Centre for high quality mostly unaccompanied singing. There's often a bit of singing at the Bowling Green (pub).

As for Whitby, there's always something going on in the Station. You could also check out Jo and Mo's afternoon singarounds (I'm not sure where they are now) and informal pub singarounds in Middle Earth, Fleece, Endeavour, First in Last out and possibly other spots.

I don't think there are themes, or if there are it might be a spur of the momemnt thing. I hope you have a good time and find what you want.