A lot of people in this thread seem to be under the impression that unitary states are always ultra-centralised affairs. If anything, a hypothetical unitary government would probably be less centralised, because there would be more focus on local government (the most important layer), which are often neglected by the current federal set-up. (I support federalism though, but if the other option is confederalism ...)
Yes, unitary just means the national government has the last word... but like the UK, you can have highly-decentralized regions like Scotland with its own Parliament.
The difference between a federation and unitary as no constitutional limits to the unitary government vis a vis the subnational units, is that what you mean?