You should leave your co.uk site alone and redirect the .com to that site.
From memory:
Point the .com DNS to the location of the current .co.uk server using the control panel where your .com is registered.
Then add the following to the current .htaccess file only in the root of the current .co.uk server. Replace domain with your domain name (before the .co.uk). Note .htaccess is normally hidden and create one if one isn’t present. Back it up before making any changes.
RewriteEngine on RewriteCond %{HTTP_HOST} ^domain\.com$ [OR] RewriteCond %{HTTP_HOST} ^www\.domain\.com$ RewriteRule ^/?$ "https\:\/\/www\.domain\.co\.uk$\/" [R=301,L]
What this will do is to redirect www.domain.com and domain.com to www.domain.co.uk in the existing server location.
Note there are lots of version of this htaccess redirect so search around if this doesn’t work or is not what you want.
Your question about a new site is a bit ambiguous. The above is for 1 site, i.e. the current .co.uk site, that any users who enter .com will be redirectd to.
If you are looking to create different content for the .com site then you will need a second new .com site.