I second this - don't get it from Howdens. A decent independent stonemason will provide better value for money.
We have granite which has a join. It did worry me before I had it put in but now I don't give it a second thought - it became a nonissue. You can hardly see it. However, this is on a work surface. Our Island is 3Mtr long which was the length of the slab we used, so no join. I wouldn't want a join on an Island.
The Howdens product you linked to is 1.9 Mtr which I suspect is why you need a join. Again, a local stonemason should be able to easily supply up to 3Mtr but maybe not in exactly the product Howdens are selling.