There is at present no indication of when this functionality is likely to be rolled out in the UK and indeed worldwide however it surely won't be long until we all start to observe similar instant preview results in our browsers.
No "instant" results until after you have conducted a search?
What is interesting is that unlike instant search results from Google's homepage, the current behaviour of instant search on shopping results is different. If for example you type "retro Gameboy" into Google's homepage with instant search enabled, the page will automatically update and jump straight to the search results pages. With Google Instant shopping however, if you type anything into the shopping homepage, you are merely presented with Google's auto-suggest feature and NOT instant search results:
Instant results will not work until you have hit enter to conduct your search and then started to amend your search query on the results page.
A bug perhaps? In fairness to Google, their product search page does state that it is in beta however this is certainly not as "instant" as it stated to be. That said, at Receptional our experience of Google Instant has been a relatively poor one with the instant results more often than not either failing to appear at all or having provided us with a page of instant results, the page has decided to just spontaneously disappear to a blank page.
Personally, I'm not a fan of any of the instant search features and Google will certainly require a bit more of a marketing push towards me before I am taken in by it.