Views Eposed filter - proximity search, include content without a postal code
If you can not do it with the 'And/Or' option on the filters (accessed by clicking the little arrow next to the filters in the views ui). This is the best option.  

Or you could use hook_views_query_alter to add something like "or where postcode = '' ".
I can not really go into detail of adding to the query as I dont know the structure of your query but you can take a look at the database api or search google.