Google provides a ranking boost to local domains. So .co.uk domains get a ranking boost for searches made in the United Kingdom. Reason being that local domains

Google provides a ranking boost to local domains. So .co.uk domains get a ranking boost for searches made in the United Kingdom. Reason being that local domains are more likely to return a relevant search result. For example if you searched for 'Google Pixel Review' in Google UK you're likely to see .co.uk sites appear at the top of the search results as the reviews may contain UK release dates and pricing that's more relevant than a US site. .io is a country top level domain assigned to the British Indian Ocean Territory, so having a .io domain will only give you a ranking boost for that specific country.