The site that is searched is specified in the HTTP header X-EBAY-SOA-GLOBAL-ID. The value of this header is not a unique integer, such as 3, but a unique string, such as EBAY-US. To search the UK site you need to make the two below changes to your code and remove any reference to X-EBAY-API-SITE-ID.
public static String targetSiteid = "EBAY-GB";
client.getAsyncHttpClient().addHeader("X-EBAY-SOA-GLOBAL-ID", targetSiteid);
The eBay docs provide a complete list of HTTP headers and a table of site IDs mapped to global IDs.