Let's break this down, slowly. And I'll put it in bold text just to emphasize the stones Pontius has displayed in doing so - he turns Prince Charles into an action hero - complete with witty catchphrases! Charles brandishes a 9mm assault rifle, gives "a tramp" the finger and also engages in repeated saucy banter with Camilla.
It goes without saying that this isn't, strictly speaking, a sequel. It uses some of the same characters, but the tone and style is so markedly different that there really isn't a comparison which pays appropriate justice to both this work and its inspiration.
It's essentially an engaging action thriller in which characterization of the royal family is a complete work of fiction. In fairness however, such Royal personality (if any exists) must be inserted either through surgical or imaginative means. Pontius is obviously bound - to set the novel any further forward in time means for difficulty painting a believable future landscape. However, it would also allow for completely fictional protagonists rather than inserting real people who may be delivering ill-fitting lines.
The pages will keep turning. You're never sure what to expect on the next page, aside from the occasional familiar set piece. Future King is an amusing read. Tennis Balls.