Is it possible to replace content on every page passed through a proxy similar to how mod_rewrite is used for URLs

Is it possible to replace content on every page passed through a proxy similar to how mod_rewrite is used for URLs? The documentation on substitute is not clear.

I have some pages I am reverse proxying that have absolute paths. This breaks the site. They need replacing and tools like mod_rewrite are not picking them up as they are not URL requests.

<VirtualHost *:80>
    ServerName  servername1
    ServerAlias servername2

    ErrorLog "/var/log/proxy/jpuat_prox_error_log"
    CustomLog "/var/log/proxy/jpuat_prox_access_log" common

    RewriteEngine on
    LogLevel alert rewrite:trace2
    RewriteCond %{HTTP_HOST} /uat.site.co.jp$ [NC]
    RewriteRule ^(.*)$ http://jp.uat.site2uk.co.uk/$1 [P]

    AddOutputFilterByType SUBSTITUTE text/html
    Substitute "s|uat.site.co.jp|jp.uat.site2uk.co.uk|i"


    ProxyRequests Off

    <Proxy *>
            Order deny,allow
            Allow from all
    </Proxy>

    ProxyPass / http://uat.site.co.jp/
    ProxyPassReverse / http://uat.site.co.jp/
</VirtualHost>

Neither of the above works at replacing the HTML string

<link href="//uat.site.co.jp/css/css.css

with

<link href="//uat.site2uk.co.uk/css/css.css

Conf after changes:

<VirtualHost *:80>
    ServerName  jp.uat.site2uk.co.uk
    ServerAlias uat.site.co.jp
    ErrorLog "/var/log/proxy/jpuat_prox_error_log"
    CustomLog "/var/log/proxy/jpuat_prox_access_log" common
    ProxyRequests Off
    <Proxy *>
        Order deny,allow
        Allow from all
    </Proxy>
    ProxyPass / http://uat.site.co.jp/
    ProxyPassReverse / http://uat.site.co.jp/
    AddOutputFilterByType SUBSTITUTE text/html
    Substitute "s|uat.site.co.jp|jp.uat.site2uk.co.uk|ni"
</VirtualHost>