lol, you know what computers are like, if you wait another couple of months there will be a 16tb for £150
Where do you stop? You have to bit the bullet sooner or later, these deals in PC bits, are rarely deals are they? ebuyer etc, reduce the price of older ones, when the new ones come out. Want bleeding edge, pay a premium, if you're happy with not the latest and greatest than more often than not the older models are ok.
especially with external hard drive, where you're buying it to back up/store files, rather than constant use for games etc, so just buy something, and then don't look at them for a year or so or you'll kick yourself for not waiting.