Not sure I understand the issue. The way you have the script at the moment, QlikView will "join" the data for you (you will see the join in your data model). If you want to "merge" the tables in to one, then you need to physically join them... you could try a simple LEFT JOIN.....
Postcode, Longitude, Latitude
This will create one table (TABLE B) with 3 columns, with only Long and Lats from TABLE A where there is a Postcode in TABLE B.
If you want to kep the Long and Lats in seperate table of their own you do not need to do anythying. The fact that you have the Postcode column named the same in both tables, QlikView will "join" the data for you.
If you want to reduce the dimension (Long and Lat) table based on the postcodes in the postcode table, you can use a KEEP join.