d747,
There is no set order in which you have to do the ratings/qualifications - you don't need to have a night rating to begin the IR - you don't need to do the IR before the twin rating and so on.
Each qualification has a set of pre-requisits, detailed in LASORS - search the CAA website to download a copy.
In essence - you do a class rating or a type rating. You need a specific type rating for anything with a maximum take off weight above 5700Kg or that has a jet engine (some exceptions). For each type within a class (Single engine, multi engine, land, sea....) you would be expected to do training as required to be safe. If there are differences that you're not familiar with (tailwheel, retractable gear, CSU/Variable pitch prop, presurisation, single lever power, EFIS/Glass) then you will also need to be trained to fly with these differences.
Flying a Spitfire - long road and lots of money, but yes it can be done on a PPL. (Guessing) there would be very stringent insurance requirements in terms of number of hours on certain types before they'd cover you to solo one.