Thats a real shame, because technically that makes the search almost useless for UK users.
The problem being the first part of a UK postcode is simply the area and the second part the more specific location in that area. So if a user searches their London postcode for example then the results will not be whats near to him at all, in fact it would just be a list of London results. As you can imagine this is useless for the user because the results will be random and they cannot see whats nearest to their location at all.
The following Wordpress plugin actually works for the whole postcode;
http://wordpress.org/extend/plugins/...-your-nearest/
The dev made it a while ago and has not updated it since but it does work with the full postcode, maybe this could be useful to you in fixing Vantages issue?
I really want to buy this theme and its perfect but for UK users the postcode not working is a massive flaw.