Do you have to be British to get panto? We sent a French critic to three shows full of cross-dressing dames and adults playing cows. Will she ever forget the experience? Oh no she won't! Standing in the audience of the Southwark Playhouse's Potted Panto, I am in Cinderella's carriage, waving imaginary reins furiously with two dozen strangers. We all swerve to avoid a giant spider on a stick, but the water guns reach …