I've been faffing around for a couple of hours trying to install the 8.1 preview without success. As a last resort (!) I did a web search and came across the PC Advisor site which mentioned ...
"Please note, if the update gives you an error message saying "the update is not applicable to your computer", then this most likely means your PC isn't using one of the languages supported by the Preview. Right now that's only Arabic, English (US), Chinese (Simplified), Chinese (Traditional), French, German, Japanese, Korean, Portuguese (Brazil), Russian, Spanish, Swedish, and Turkish."
So it looks like trying to install the 8.1 preview on a system with English (UK) (which is probably most people reading here) is going to fail.
Thought I'd mention it here as it might save someone a bit of wasted time.