On the surface it doesnt seem to make sense to have this configuration. But imagine you live in the UK and you have a very limited choice of company cars all of which must be less than 2.0 litres. Imagine the sales BMW would make if they could get on that list, so they bring out the 518 and most employees choose it, de-badge. I also believe that this model was introduced due to small engine tax breaks in other parts of Europe. I find that a 530 struggles with the weight so can't imagine how a 1.8 handles it.