I'd second what MattUK says.
I'm pretty sure that Lightroom 3.5 is the version of Lightroom 3 you'll get if you purchase a full product now. There is certainly a free upgrade from 3.x to 3.5 available here
http://www.adobe.com/support/downloa...jsp?ftpID=5256
Adobe don't drop prices because of any imminent release, if they have announced a new version, you should be entitled to a free upgrade, if you purchase between the announcement date and the final release of the product.
If you purchase a boxed product for 90 pound, I'd suspect that it's a student and teacher edition, for which you will have to fax proof of your eligibility (NUS card, recognised school letter headed paper etc.) to Adobe, before they will send you a licence number. Alternatively at that price, it might be from the USA and the reseller will provide you with a activation number, but you will only be able to reinstall it a maximum of 3 times.
I hope this helps
Regards
Nick