Basically, the higher the RON rating, the better controlled the burning of the petrol. This means that the ECU can be more exact about when to ignite the fuel to get the best (cleanest) possible burn. This didnt use to make that much difference before VVT, but now the engines can adapt to the fuel you give it and detect knock and adapt to that, the higher the octane the cleaner the burn. The cleaner the burn the better MPG you will get. The car is natively (ie in Japan) desgined and mapped for something like 100RON. The closest we originally had over here was 98RON so thats what they recommend. It will adapt to higher RON though, so 99+ is the best. 95RON means it has to retard the timing which gives a worse burn and lower MPG. This is all in very small numbers of course, but when I tested it, Tesco 99RON gave me more MPG than the extra cost per mile over 95RON.
Now apart from the RON rating, other big brands (Shell. BP etc) put in additives. These help clean the engine and reduce deposits of crap in the engine. In theory this should also help the car run smoother and cleaner.
In short, if you can get Tesco 99RON, do it. If not, then try optimax etc and see which is best for you