https://wpbarista.com/force-wordpress-ssl-https/

http://www.wpbeginner.com/wp-tutorials/how-to-fix-error-too-many-redirects-issue-in-wordpress/

https://managewp.com/blog/wordpress-ssl-settings-and-how-to-resolve-mixed-content-warnings

The links above are pretty good so follow them but I would use the following amendment to the .htaccessfile…

# HTTP to HTTPS redirect
RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://alliance-systems.co.uk/$1 [R=301,L,NE]

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Make sure you put the rewrite section ABOVE the default WordPress section.