I reckon the reason to use the higher octane fuels available in the UK in a classic is the reduced amount if any of added ethanol, which prematurely ruins some of the fuel system components.
This is the main reason I use either V-Power or BP Ultimate. Also as my cars are not daily drivers these fuels have a longer shelf life and are less likely to go off while the car is left in the garage for a few weeks. It is the high ethanol content of modern fuels that are causing problems for classic cars and it is going to get worse as more ethanol is introduced.
I even used V-Power in my '63 Super Delux as I found that using regular 95 Octane, with Valvemaster added, would cause the engine to pink (pre-ignite) going up hills.
