We have a site primarily targeted at the UK market, and are adding a US-market alternative. As per Google's recommendations:

To indicate to Google that you want the German version of the page to be served to searchers using Google in German, the en-us version to searchers using google.com in English, and the en-gb version to searchers using google.co.uk in English, use rel="alternate" hreflang="x" to identify alternate language versions.

Which gives us:

<link rel="alternate" hreflang="en-gb" href="http://www.example.com/page.html" />
<link rel="alternate" hreflang="en-us" href="http://www.example.com/us/page.html" />

We do get enquiries from other areas of the world - particularly where there are expat communities (Dubai, UAE, Portugal etc). By adding the above tags, is there a risk that Google will only surface our site for UK and US search users? Do we need to specify a catch-all that will default all other searches to our UK site?