Instead of just paying extra £400 for a 512Gb SSD or extra £800 for a 768Gb one, you're forced to pay additional £100 for an upgraded 2.6Ghz CPU. It's not the worst price and certainly not a disaster, but you don't have other options even if you don't need faster CPU. In other words, see upgrade option for a 2.6Ghz model and compare prices.
What's that go to do with the original point of storage? £400 for a 512GB SSD v £200 for a NAS 2TB Storage which a load of devices use? Seriously, read the question.