You need to be careful when comparing modern RON rated fuel with the old octane system or the later STAR system.
RON does not equate to octane or star
This is why you shouldn't dice in and use the octane ratings when timing your vehicle today.
The manuals were written years ago and RON wasn't invented.
Modern fuel (sometimes called "petrol") is not what is was when Series 2s first appeared.
What's sold now is quite low in octane value, nearer to "pool" petrol of yesteryear.
.... and this is why we are having problems with the dilution of petrol with ethanol (ethyl alcohol).
The premium fuels are a little better.
You need to do some road tests and calculations to see if the premium fuels are better for your vehicle.
Regarding diesel, the premium diesels have a cetane booster (petrols equivalent of octane rating).
On a good run I notice a little more power (easier when running up gradients) and smoother running.
Whether the extra cost justifies using premium diesel all the time - I don't know.
You can use a diesel centaur booster additive like Miller's Sport Diesel.
I'm currently using this, bought from eBay.
A few mls per tankful, it goes a long way.
I also put the occasional squirts of a mineral 2-stroke oil in - using an old Redex UCL squirter that garages used to have.
Regarding the price of fuel:
The prices seem to change every day.
Nationally Asda have said they will be the cheapest when up against local competition.
This is confused by Sainsbury's and Tescos occasionally giving fuel discount vouchers away when you open over a certain amount.
You need your wits about you.
To that end I subscribe and use this online app
Petrol Prices.com put it on your phone.
Very useful for when you are away from home.
:petrol_pump