The Spitfires increasing in value are the two-seaters now they can do 'experience' flights legally and charge for them. Hawker Restorations have even converted a Hurricane to a two-seater, which is based at Biggin Hill.
Before, they were converting two-seaters back to single seat configuration, now the reverse is true.
Of course, Mustangs have long had a jump seat in place of the rear tank.