Here are a couple of thoughts to help with your list, keeping in mind the aircraft/armor that defined that era for your countries:
Cold War:
USA - Aircraft: F-4 Phantom, used by Navy, Marine Corps and Air Force. Interceptor against the Soviet Tu-95's, fighter bomber in SE Asia, over 5000 built, etc.
USA - Armor: Something along the lines of either the M113 (not sexy, I know) or possibly the M60 Patton as it's service history (1961-1997) fits the Cold War era almost completely. Plus all the bonus coverage of the M60A3s in service with Egypt getting some press.
Soviet Union - Armor: Either a T-62, T-72 or BMP variant. Personally I would go with a BMP as it defined the role of mechanized infantry in the Cold War, as well as the radical design that provided almost no profile to target.
Soviet Union - Aircraft: MiG-21bis, possible the Su-17/22/22M family. The main rationale for using either of these designs are the sheer numbers of these two airframes that were exported. Remember, it was a pair of Su-22's that were shot down in 1981 by the F-14 in the Gulf of Sidra. I would consider either the iconic aircraft of the era for the Soviets.
Great Britain - Aircraft: Harrier GR.1 or FRS.1. The signature British aircraft of the Cold War (well, maybe second to the Avro Vulcan, but at 1/48th scale, that would be a BIG plane). Instantly recognizable around the world, combat tested in the 1982 Falklands campaign. Other options include mainly consortium aircraft, such as the SEPECAT Jaguar or Panavia Tornado, but neither are purely British in nature.
Those would be my suggestions for some Cold War era stuff. Hope it helps.