£65 for 350 miles is about right, unless they are all motorway miles, and changing the grade of fuel will have no major differences to this..
You should be using Shell V Power at a minimum. Its 99ron fuel and is the closest to the 100ron available in Japan. My old one which was mapped for Japamese fuel was always run on Optimax, which was 98ron, and an octane booster which brought it upto 100ron.
Personally now that V-Power is99ron I think that this should be fine for normal driving (Although this is just my opinion) and only need octane booster if you are going to boot it.
Using a lower grade of fuel can cause the engine to knock (Which basically means the the fuel ignites at the wrong time in the combustion cycle) which results in bang and goodbye engine.