htaccess - redirect entire domain to specific url on new domain

Question

I've setup a number of htaccess rules to redirect a large number of old domains to one new domain. Each old domain points to a specific page on the new website. For example:

olddomainlocation1.com redirects to newdomain.com/location1

olddomainlocation2.com redirects to newdomain.com/location2 etc.

The rewrite rule redirects as expected if you visit an old domain name without any path in the url, but when I visit an old domain with a path in the url it doesn't work.

eg. olddomainlocation1.com/index.html, my rewrite rule doesn't work, I just end up on newdomain.com/index.html (which doesn't exist)

Here is an example of my htaccess rule. Any ideas where I'm going wrong?

<IfModule mod_rewrite.c>
  RewriteEngine on
  RewriteCond %{http_host} ^www.olddomainlocation1.co.uk [NC,OR]
  RewriteCond %{http_host} ^olddomainlocation1.co.uk [NC]
  RewriteRule ^(.*)$ https://newdomain.co.uk/location1/ [R,NC,L]
</IfModule>

UPDATE

I have updated the rule now which seems to be closer to working, but it's not quite there yet.

<IfModule mod_rewrite.c>
  RewriteEngine on
  RewriteCond %{HTTP_HOST} ^www.olddomainlocation1.co.uk$ [NC,OR]
  RewriteCond %{HTTP_HOST} ^olddomainlocation1.co.uk$ [NC]
  RewriteRule ^ https://newdomain.co.uk/location1/ [R,NC,L]
</IfModule>

Adding $ to the end of my RewriteCond makes sure the rule runs on pages like /index.html.

When I test this rule on https://technicalseo.com/tools/htaccess/ and it's telling me that the redirect should be in place, but when I test the redirect, the url isn't changing for me in my browser.

UPDATE 2

Further update: I do have WordPress specific rules above the domain redirects, which could be causing an issue, here is that code:

# BEGIN WordPress
<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteCond %{HTTP_HOST} ^newdomain.co.uk$
  RewriteRule .* - [E=HTTP_AUTHORIZATION:%{HTTP:Authorization}]
  RewriteBase /
  RewriteRule ^index\.php$ - [L]
  RewriteCond %{REQUEST_FILENAME} !-f
  RewriteCond %{REQUEST_FILENAME} !-d
  RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Answer 1

Further update: I do have WordPress specific rules above the domain redirects, which could be causing an issue

Yes, that is certainly the problem. The WordPress specific rules (ie. the WordPress front-controller) should appear later in the file. The external redirect needs to go near the top of the .htaccess file, before the WordPress front-controller. Order matters.

The problem is that a request of the form /index.html (which does not exist) is rewritten to the WordPress engine before your redirect (that appears later) is able to process the request. So this will ultimately trigger a WordPress generated 404 instead of a redirect.

For example:

# Redirect olddomain to page on newdomain
RewriteCond %{HTTP_HOST} ^(www\.)?olddomainlocation1\.co\.uk [NC]
RewriteRule ^ https://newdomain.co.uk/location1/ [R=302,L]

# BEGIN WordPress
# :

Aside: I've combined your 2 conditions into a single RewriteCond directive.

You do not need the <IfModule> wrapper nor repeat the RewriteEngine directive.