I'll give it a shot, k. The five current Londons---the Maroon, Gold, Super Gold, Jubilee, and Reference, all share the same identical internal design and structure (how could it be otherwise?---it's what makes a London a London!), but differ in the quality of the materials used to make them, and in their assembly tolerances. The internal wire and coils in the Reference, for instance, are of a higher grade than in the lesser models, and the magnets are of the more expensive rare earth variety. The stylus profiles differ amongst models as well, the Maroon being Spherical, the Gold Elliptical, the Super Gold and Jubilee Extended Fine Line, and the Reference Ultra Low Mass Fine Line. Other than that, the difference between models is in the housing the internal guts are installed in.
The Maroon, Gold, and Super Gold share the same stamped tin housing of the Deccas of the 70's, along with the old Decca mounting bracket, a real pos! The bracket is a somewhat flexible red block of molded plastic, the top of which is bolted to the head shell. The cartridge slides onto the bracket, the electrical signal from the cartridge passing through electrical contacts in the bracket to pins on it's rear for your arm wires. It's a terrible mounting and connection design, but you don't have to put up with it. London offers the Decapod, an aluminum block which replaces the bracket, and is installed as the top of the cartridge at the London factory. DO NOT BUY A LONDON CARTRIDGE WITHOUT THE DECAPOD!
As for the tin housing, Decca users have long applied various substances onto it's exterior in an attempt to lessen the resonances it exhibits; modeling clay, Blu Tack, Sorbothane, etc. Even with the resonances of the housing addressed, the cartridge is somewhat microphonic, one reason the Townshend Rock turntable is a favorite with Decca/London enthusiasts---the Rock's damping system eliminates that microphony remarkably effectively, making it a non-issue.
The Jubilee was developed to address the issue of the sonic consequences of the tin metal housing. It's more solid, less resonant/microphonic structure allows more of the Decca design's potential to be realized, and is considered by some (Warren Gregoire, for one) to offer more of an upgrade from the Maroon, Gold, and Super Gold, than the Reference does to it. The Jubilee's housing eliminates the need for the Decapod as well, having threaded mounting holes in it's top surface, and the normal cartridge pin design.
The Reference really brings the Decca design into the 21st Century. A very stylish machined aluminum body/housing, like other top cartridges. Superior parts and assembly tolerances, hand built by John Wright, the designer of the Reference, and owner of London.
So which London to get? As always, it's a question of system balance, the most effective allocation of your Hi-Fi Dollars/Pounds (in honour of the Britishness of Decca/London ;-), and, most importantly, of course, the other parts of your record player. There is no point in spending the extra money for the Reference if your pick-up arm won't allow it's superiority to the Jubilee to be heard. The great thing is, no matter which London you choose, it will have the Decca/London sound, unlike any other cartridge!