The differences don't break down cleanly by model year, but it's roughly like this:

1961-63 = P1800, B18 motor, SU carbs, 100 HP, built by Jensen in England. These are the first 6000 cars, also sometimes called 1800A. Bullhorn bumpers, interior upholstery and some trim are different from later cars. This is the only model correctly called "P" 1800. Heavy "egg crate" grille.

1964 = 1800S... same as above, but built in Sweden. Interior transitions to the later style early, power increases to 108 late in this period.

1965-66 = 1800S, straight bumpers, power increases to 115 for '66. Cheap-looking stamped stainless grille.

1967-68 = 1800S, side trim no longer follows the body curve but runs straight, door handles fair into the tailfin chrome. Improved rear suspension. Nicer multi-piece stainless grille. '68 gets headrests and side reflectors (I think).

1969 = 1800S, B20 motor with Stromberg carbs, 118 HP, "year only" rear brakes. The lowest production year, only about 1500 built.

1970-71 = 1800E, B20 with Bosch D-jet fuel injection, 130 HP, rear disc brakes, fake wood-grain dash, different instrumentation, center console, styled wheels, different badging, taller gearing. Stainless grille now painted black (may have been on the '69, too -- ???).

1972 = 1800E, mechanically as above with lower compression, 112 HP, high-back seats, different upholstery on doors as well. The last of the coupes.

1972-73 = 1800ES, the Sportwagon. Mechanically identical to the '72 E, except with yet taller gearing.