Tate Britain, so it is my default 'need escape from the house' place to go. About 1 hour before closing is the best time to be there as it so quiet and sleepy you can potter around undisturbed; the total opposite to the hyper frenzied experience of Tate Modern. At the moment they are waiting to do the major new rehang and in preparation have cleared all the art out of 5 or 6 of the main galleries. It is so unusual to get to wander round empty galleries and it makes you appreciate what beautiful spaces they are – plus it really adds to the peaceful chilled out atmosphere (perfect when you need to do some serious thinking!).