Wouldn't it have been better for the Order to just flee Britain as soon as Harry turns seventeen, since I don't think anyone in the Order knows that Harry is hunting Horcruxes anyway?
Running away isn't the Order's style, nor would it have helped in the long run.
The whole point of the Order of the Phoenix is that they're not the people who accept Voldemort's rule: they're the ones who fight against him, who risk their lives for something worth doing. Fleeing the country to escape Voldemort doesn't fit their MO. They're the kind of people who stay and fight to the bitter end for what's right. Running away smacks of cowardice, which just isn't their style. Without them, who would have continued the fight? Who would have protected the Muggles and Muggle-borns of Britain from the Death Eater tyranny?
Plus, if they had fled the country, Voldemort would have had all the power and no serious opposition. After consolidating his hold on Britain, he would surely have moved to conquer the rest of Europe and the world. By fleeing, the Order would only have been putting off the inevitable - and by the time he had Britain more firmly under his thumb and was ready to move on to other countries, Voldemort would have been even more powerful, and they would have had even less of a chance of stopping him. Staying in Britain was their best, albeit slim, chance of defeating him: to catch him while his rise to power was still recent and not yet consolidated.