Take care not to sit in the front row for Walking on Sunshine. You might be killed by this British musical’s glee tornadoes. Think of Mamma Mia!; change the specifications from Abba songs to 1980s pop hits; unleash the all-singing, all-grinning cast. The overkill is almost compelling. Like a storm-follower you want to sit right in its eye. But be warned. The script, unlike Mamma Mia!’s, is dire. The choreography is über-camp. The excess exuberance, from everyone in sight, is nearly nuclear.