Look lads. here is what u do to buy and use a upgraded version of pro:
1. download the win 8 preview/demo
2. install it.
3. load up the upgrade assist and follow instructions on purchasing pro for £25
4.once purchase , download the upgraded files and dump them on a usb stick or whatever
5.install the badboy and it will activate instelf.
DONE
Got this info from various sources who have tried this approach.
Some are getting it for like £15 via quidco etc
edit:
There is one small catch and i will edit my post,
the catch is that if u have to upgrade your hdd or do a reformat and install, you need to do the whole steps again. the issue is this, win 8 preview will run out on feb 2013 so after that, if u want to install win 8 again, u got to get a legit key from a previous version of windows.