Whilst I know there is a guarantee - the price here on Amazon is around twice what Tesco, Dixons, Currys and PC World are listing. I am posting this on 18th July so things may have changed in the last few days and Amazon has not caught up yet!
The professional edition was only available for about 24 hours at the price of £89.97 from the 15th July on amazon before it 'sold out'. The home premium edition was available for a few days at £44.97 before it too 'sold out' on amazon. (The prices are now £179.97 and £74.97 respectively). Go to one of the following to get it at the discounted price. currys http://www.currys.co.uk/martprd/editorial/windows7-preorder?int=1043_windows7_preorder (£89.99 and £49.99) pc world http://www.pcworld.co.uk/martprd/editorial/windows7-preorder (£89.99 and £49.99) tesco direct http://direct.tesco.com/p/inc/specials/windows7/ (£99.97 and £49.97) staples (a bit dearer than others above) http://www.staples.co.uk/ENG/static/wrapper.asp?CT=1
Just to clarify the above The initial discounted price of £89.97 is a limited offer from 15th July to 9th August OR until the discounted stock runs out. With amazon, the amount of stock they had ran out after about 24hrs resulting in the farcical situation of other retailers offering it at the original discounted price whilst Amazon have hiked there prices to the 'normal discount levels' available up until launch on 22 August. This promise to match their own future lowest price before release is optimistic at best as the price will never again go down to the original discount levels unless they can obtain more of this discounted stock from microsoft. They are basically a victim of their own success.
Anyone who managed to buy the Professional Edition at the pre-order price was lucky, including myself. I live in Germany and the German Amazon started selling the pre-order editions on the 15th at Nine in the morning, at three minutes after Nine they were completely sold out. According to the PC Magazines here, Amazon was only given 1500 Copies of Windows 7 Home Premium to sell cheaply, two other Shops were given 500 each. There was no reduced pre-order price for Windows 7 Professonal Edition (which sells here for about 270 Pounds) because Microsoft did not offer it. The whole Advertising Campaign with the pre-order Editions was nothing else than cheap Publicity trick from Microsoft, so why not complain to them and not Amazon. P.S. Last Month in America Amazon was given what seems to have been an unlimited amount of pre-order Copies (all three Editions-Home,Professional and Ultimate) and offered them cheaply right up to the end of the 4 week offer, at prices that were a lot cheaper than in Europe.