London greets us with music in the air and a skyline that feels like a promise. We land at a rock-and-roll hideout in Soho—formerly MI5 research space, now a boutique sanctuary curated by Mark and Shireen Fuller—where portraits of Hendrix and Tyler watch over the lifts and the rooftop spins a perfect soundtrack. It’s our launchpad for a fast, joyfully chaotic plan: a bullet train to Liverpool for Strawberry Field and Penny Lane, the Beatles’ childhood homes, and a hopeful stop at the Cavern C...