My G/S will get me just over 600 km of gentle driving, which would probably get you from a servo to the next one in most parts of the UK, or Australia for that matter.
But I fill up at my local oil company owned station on the way home every day, and it is a nice feeling to point the old girl down the road in the morning, just follow the front wheel and know I dont have to think about petrol before my lunch stop.
Petrol here is, er, variable, in quality, so if I have to top up on the road I try not to let the tank get below half full, just in case the country stuff is crap, so a big tank cuts down the stops , and keeping the tank relatively full also helps to offset the weight of a full load of luggage on the back too.