[1] To do so, you start with the base xs:string type and impose a restriction on it. The restriction is best through a regular expression. Then you find a handy regexp for the dd/mm/yyyy pattern with more or less sophistication, depending on how coarse or how fine you are happy enough.
[2] This is one of the realization of the above appraoch.
[tt]
<!-- arbitrarily called uk_date for date of dd/mm/ccyy format -->
<xs:simpleType name="uk_date">
<xs:restriction base="xs:string">
<!--
ref [ignore]
[/ignore]
an entry accredited to Dany Lauener
-->
<xs

attern value="(((0[1-9]|[12]\d|3[01])\/(0[13578]|1[02])\/((1[6-9]|[2-9]\d)\d{2}))|((0[1-9]|[12]\d|30)\/(0[13456789]|1[012])\/((1[6-9]|[2-9]\d)\d{2}))|((0[1-9]|1\d|2[0-8])\/02\/((1[6-9]|[2-9]\d)\d{2}))|(29\/02\/((1[6-9]|[2-9]\d)(0[48]|[2468][048]|[13579][26])|((16|[2468][048]|[3579][26])00))))" />
</xs:restriction>
</xs:simpleType>
[/tt]
[2.1] Note that you don't put anchors ^ and $ to the pattern in the schema, you don't need to.
[3] Then you set up the element "navdate" like this.
[tt]
<xs:element name="navdate" form="unqualified" type="uk_date"/>
[/tt]
and things should be set.