Have the country in craft.commerce.countriesList default
Not particularly proud of this code, but if you want to do it from Twig, something like this:

{% set countryList = ['United Kingdom', 'United States'] %}

{% for country in craft.commerce.countriesList() %}
    {% if country != 'United Kingdom' and country != 'United States' %}
        {% set countryList = countryList|merge([country]) %}
    {% endif %}
{% endfor %}

&lt;ul&gt;
    {% for country in countryList %}
        &lt;li&gt;{{ country }}&lt;/li&gt;
    {% endfor %}
&lt;/ul&gt;