I think what it boils down to, is that if you are a business and you own a website built on a .co.uk domain name, you cannt afford to let someone come along and take the .uk version of your domain name.
By letting the .uk version go to someone else, you are leaving your business open to be abused as the .uk will look like it has more authority being shorter and people will confuse the 2 domains.
The problem is the majority of business owners won't even have the option to secure the .uk version of their domain, they will either be beaten their by trademark holders or they won't know about the auctions so other people will register their corresponding domains.
Nominet would'nt be able to email the millions of businesses to let them know that the corresponding domain was being auctioned. This is because nominet have said the .uk domains are a totally seperate product, so would have no relevance to businesses owners with .co.uk domains or .org.uk organisations, so couldnt email them about it. This would mean, because only domainers would know about the new .uk domains, that they could register the majority of the new domains when they are released.