A dimensionally correct UK plug to use as a template in your designs.

Use as a template.

Whilst it will probably print on a 3D printer without any issues it would be of limited use other than blocking a plug socket much like the socket safety covers that are sold to prevent children sticking objects into the holes.

The socket safety covers have been proven to cause more harm than good. 99% of all UK sockets sold in the past 60 years have inbuilt safety mechanisms that prevent the live and neutral holes from being exposed without a plug been inserted so these covers are not required. See http://www.fatallyflawed.org.uk for more information about this.