I'm on Sky and can confirm they block thepiratebay.se, I'm not sure about .is, I'll check once I'm home but over SSH it seems to fail.

I'm with o2 broadband and they do, http://thepiratebay.se/ gives "The page you're looking for has been blocked."

They most definitely do. Sky here, and I can't access thepiratebay.se or thepiratebay.is.