I wondered that too, especially as Frenesi is owned by the same people and that got shipped over. Then I read this from a man in the know over on the Key forums:
"It was just sold to Dan Friedkin/Comanche Fighters last month (just a few weeks ago) - it had previously been owned by Max Chapman. There of course wasn't going to be enough time to have the aircraft shipped over to England in-time for Flying Legends, so that is why the trans-Atlantic flight quickly came about. "Berlin Express" and "Frenesi" are two of eight flying Mustangs in the Comanche Fighters stable - an A-36 "Apache", P-51B "Berlin Express", P-51C "Princess Elizabeth", P-51D "Frenesi", P-51D "February", P-51D "Double Trouble Two", P-51K "Fragile but Agile", and TF-51D "Bum Stear"."
That's quite a collection of 51s!