Drop-down and checkbox in a condition
A checkboxes field returns an array of models, that you can inspect using {{ dump(sourceCountry) }} in your loop.

The model has a __toString method that outputs the checkbox value when you output the model using {{ sourceCountry }}. This magic method doesn’t do its thing when you use the model in a comparison operator, so you are currently comparing a model to a string.

Access the value property directly and your conditional should work.

{% if sourceCountry.value == 'GB' %} 
    {% set visibility = true %}
{% endif %}