I basically agree with you. (I saw it last month). However one thing that made it worthwhile for me was that its setting was a West End theatre - and this was essential to the show. Seeing it IN a West End theatre gave it resonance. Additionally, looking around the largely local audience, you could see that if they weren't alive during the blitz, they certainly had grown up hearing firsthand stories about it.
There were slim pickings for shows that aren't Broadway transfers or dubious hen party jukebox musicals. I was looking for an English show and it filled the bill nicely.