I wouldn't play around with the .co.uk when the .com has great rankings. I think it's enough to change the links internally, and not move the site to a new domain as well.
So, why not rebuilding the .com on WP and then 301 the current .php pages to the new WP pages plus redirect http to https?
You might even want to keep the .php file extension. Should be easy to do that, at least for posts:
Log in to your WordPress admin dashboard
Navigate to Settings > Permalinks
Select ‘Custom Structure’ under ‘Common Settings’
Add .html extension to your desired URL, for ex. /%year%/%monthnum%/%postname%.php
Save the settings
(changed from: wordpress.org/plugins/add-html-extension-to-pages/ )
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