>>>does it really matter if i have a .com / a .org / .biz domain ?
.org was always intended to be used by non-commercial groups, such as charities. However unlike (say) .gov or .ac.uk there aren't any actual restrictions as to who can use it. Even so, it still looks 'wrong' to many people when used on a commercial website, so I'd advise against it.
While there's nothing wrong with .biz either, it seems to be used almost entirely by very small firms (such as local florists or tradesmen). It doesn't really announce 'Hey, you're dealing with the big boys now", so I'd also advise against using it if you're planning on trading nationally or internationally.
.co.uk is used by many large businesses that trade solely in the UK (or for the UK operations of international firms). So, for example, Argos, W H Smith and the UK operations of e Bay, Toys R Us and Barclays Bank all use .co.uk. So if you only intend to trade within the UK (or mainly within it) .co.uk is absolutely fine.
.com is used by businesses based in the USA (e.g. eBay.com) , businesses that trade internationally and also by many UK businesses, both large and small. (e.g. Tesco.com).
.co.uk is a little cheaper to acquire than .com but, within a business plan as a whole, the difference is really negligible. It's simply a matter of choice. Indeed, many companies register both domain names in case their customers forget which one they use. (e.g. if you put Tesco.co.uk into your browser's address bar it automatically diverts to Tesco.com because Tesco has registered both domain names).