No worries at all
You are right, it is not possible to purchase .uk directly from the domainer point of view.
.CO.UK is for commercial , and .LTD.UK is for limited companies. The .CO.UK is open for all comers whereas the .LTD.UK is restricted to those with a company number.
The reason for *why* is a tricky one but basically I believe it is that .UK would be too open and further categorization was required eg. .SCH.UK for schools.
It is interesting to see that for North America they have had to use .NET (.net.uk) .EDU (.sch.uk) .COM (.co.uk) and the like to cover the same bases, and if every country in the world did similar it would be unworkable.