I've emailled them about it before and they say it's because the website usually picks by default the cheapest rate if you don't specify a discount code. If you specify the discount code (i.e. click the links KLM provide) it'll provide 30% off the standard rates which happen to be dearer than the standard web promotional rate in the UK.

So basically wherever I hire a car with Sixt, I try booking it as a FB member first and then as member of the public and then select the cheapest one.

Yes, whatever rate you use you still get miles. Make sure to put it on the booking and show it when you collect your car.

It's also worth knowing that when you're a FB gold member you're entitled to a free Sixt platinum card, the benefits seem to be the same as you get with the FB card but mine is valid until 2013 so it'll be useful if I ever lose FB status, also it allows you to save a profile on their website.

Sixt seems to consistenly be the cheapest in my experience and even before I reached gold status they were very generous with upgrades.

I've started on what's currently a badly worded description of Sixt car rental offers. Nothing much else on the site either, but I've got a very quiet December and so I'm going to populate it with some content. But currently it's a bit of a badly worded mess.