Answer: Yes, you can save that much.
£90 really isn't that much over the course of a year - about 25 pence per day.
According to this website
The price of electricity in the UK in 2013 averages 17.2 pence per kWh
That means over the course of one day, you need to save 1.45 kWh (kilowatt hours) in order to save 25 pence. You can accomplish that by ...
As a starting point, this is what I found in regards to 85 vs. 87 in Colorado:
Research several years ago from the American Petroleum Institute showed that lower air pressure at higher altitudes allows vehicles to perform as well with 85 octane as they would with 87 at lower ...