I was Wondering about Government Monopolies...

A thought crossed my mind (as they tend to do on an annual basis) as to why government departments have a monopoly on certain things.

For instance the DVLA. Why has the government got the monopoly on vehicle and driver registrations? What's to stop me, legally, from setting up my own version of the DVLA and issuing my own registration numbers, driving licences, etc? I mean, I'm sure I can offer better value for money, guarantee not to share any information with any third parties for any purposes and guarantee not to fine people for petty reasons like not updating information,  so should get a lot of custom. After skimming off administration expenses I can hand the rest of the cash over to the government, so no harm done, except maybe provide a more flexible and customer-oriented service.

So is there anything actually stated in law, that prevents people setting up agencies in competition with government ones?