The sat nav does allow entering of partial postcodes, whether the CD-ROM version or the DVD. To use it you just start entering the postcode under the "town" option. For example, it would let you enter CV23_0 but not the last two letters. Then the streets in that immediate area are displayed for you to choose either by scrolling or by entering the name under "street".
It would of course be better if you could enter an entire postcode, like the handheld units which are a tenth of the price!