Todd,
IIRC, the methodology didn't change - the fuel changed. I also think that England used a different methodology than the US for octane ratings when our cars were built. I'm not sure there is a easy comparison, all things considered.
When EPA mandated the phase out of using lead as an anti-knock agent in fuel, the ability to economically maintain the fuel octane levels that TEL (tetra-ethyl-lead) made possible went away. I think I recall in this area, leaded regular fuel was rated at 96 or 97 octane and premium was considered 100 octane. When unleaded fuel for cars with catalytic converters had to be accommodated, many stores converted the premium tanks and pumps to unleaded instead of installing new storage and pumping systems. Leaded regular did stay available until about the early 90's. It was only a couple of years later until the cylinder head in my truck was destroyed due to valve recession because of no lead in the fuel. The use of ethanol as an anti-knock agent and oxygenator in todays fuel leads to a couple of issues for older vehicles that were designed for leaded fuel. The affinity to absorb water, possible corrosion due to the water content, and fuel system materials that were never designed to be compatible with ethanol are reasonably well documented and understood. One thing that is only rarely mentioned is that the BTU content of ethanol is only about half of pure gasoline. A 90-10% mixture of gas and ethanol would deliver a fuel/air ratio about five percent lean by volume as designed into the fuel delivery system which could lead to some drivability issues.
Just some thoughts on today's fuel,
Have fun, Don