I have spent much time in the past trying to figure out the same thing. I've gone into a great length to find a job which I thought will cause least harm to any living beings and so on, but the environment is so difficult and challenging, that if I am not careful, I will keep creating bad kamma by the way I am reacting to this environment.
At one point I had great difficulty with understanding what is the right thing to do even when I had to sweep the leaves in the meditation centre which is in Asia, where the ground is covered with small animals. I thought that by sweeping and clearing the ground I will inevitably kill some animals, but on the other hand, if I don't do it, then I am just avoiding my responsibility and leaving it to other people. At the end I decided that I have to sweep the leaves as everyone else. In this case, I was able to find a compromise - gloves and a bucket in which I collected the leaves at the same pace and with the same efficiency as everyone else hoping that this is a gentler way than using a hard broom, but finding compromises like this is not always possible in all the tasks we do.
I asked the same question and I was told the jataka (sorry, I don't remember the name) about a woman who was sotapanna and was married to a hunter. She was helping him to prepare his arrows and maybe even carry them to the forest for him, without any unwholesome consequences for her, because of her state of mind (no intention to kill any living beings).
So, as the the Dhamma friends above have said, it is not possible for us to go through this life without causing any harm. I think it all depends on our state of mind. And if I had a choice between working the office in England and picking the grapes presumably somewhere nice and sunny, I'd go picking grapes