.htaccess redirect all domains, with paths, to one

Your problem is that mod_rewrite directives are not inherited by default. The mod_rewrite directives in your /members/.htaccess file are completely overriding the mod_rewrite directives in the parent .htaccess file. The mod_rewrite directives in the parent .htaccess file are not being executed when your wiki is accessed.

You can enable mod_rewrite inheritance from the /members/.htaccess file (using RewriteOptions Inherit), however, the current directives in the parent .htaccess file are still not going to work if they are inherited. (Directives are inherited as if they are literally copied into the subdirectories .htaccess file.)

To use mod_rewrite inheritance, you would need to change the following directives in the parent .htaccess file:

RewriteEngine On
RewriteBase /
RewriteCond %{HTTP_HOST} !example.org$ [NC]
RewriteRule (.*) https://www.example.org/$1 [R=301,L]

to read:

RewriteEngine On
RewriteCond %{HTTP_HOST} !example\.org$
RewriteRule ^ https://www.example.org%{REQUEST_URI} [R=301,L]

The RewriteBase directive is superfluous here. (Unless you have other directives?) The REQUEST_URI server variable contains the full URL-path, regardless of where is it used, unlike the captured URL-path from the RewriteRule pattern which notably has the directory prefix removed in a directory/.htaccess context. The NC flag should also be removed on this negated CondPattern, since you also want to redirect malformed requests.

Aside: However, this still doesn't canonicalise the www subdomain and nor will it redirect non-HTTPS requests to example.org - is this a requirement? For that you would need something like the following instead:

RewriteEngine On
RewriteCond %{HTTP_HOST} !^example\.org$ [OR]
RewriteCond %{HTTPS} !on
RewriteRule ^ https://www.example.org%{REQUEST_URI} [R=301,L]

Note the start of string anchor (^) on the CondPattern (!^example\.org$). And the additional check against the HTTPS server variable (which is OR'd).

And then in /members/.htaccess, you replace the first part (which would never work anyway, since the RewriteRule pattern matches against the URL-path only, not the hostname):

Options -Indexes
RewriteEngine On
RewriteRule ^example.org.uk(/.*)?$     https://www.example.org/$1
RewriteRule ^example.co.uk(/.*)?$ https://www.example.org/$1 

With the following:

Options -Indexes
RewriteEngine On
RewriteOptions Inherit

Alternatively, you forget inheritance and simply copy the parent mod_rewrite directives into the /members/.htaccess file:

Options -Indexes
RewriteEngine On
RewriteCond %{HTTP_HOST} !example\.org$
RewriteRule ^ https://www.example.org%{REQUEST_URI} [R=301,L]

# MediaWiki pretty URL rewrites
RewriteRule ^/?members/wiki(/.*)?$ %{DOCUMENT_ROOT}/members/wiki/index.php [L]
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
RewriteRule ^(.*)$ %{DOCUMENT_ROOT}/members/wiki/index.php [L]

Aside: However, these directives would not work in a /members/.htaccess file either?! These directives are intended for the server config (or VirtualHost) context.

In per-directory .htaccess files, the RewriteRule pattern matches against the URL-path less the directory-prefix. So, a pattern that starts ^/?members/ will never match when the .htaccess file itself is in the /members subdirectory. (However, this would work in a server/VirtualHost context since the pattern matches against the full URL-path.)

Also, in per-directory .htaccess files you cannot rewrite to an absolute filesystem path, it must be a document-root relative URL-path. (However, you could do this in a server/VirtualHost context.)

Also, %{DOCUMENT_ROOT}%{REQUEST_URI} is more typical of a directive used in the server config. It is simpler to just use %{REQUEST_FILENAME} in .htaccess.

For this to work in /members/.htaccess you would need to change these directives to read something more like the following:

# MediaWiki pretty URL rewrites
RewriteRule ^wiki(/.*)?$ wiki/index.php [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ wiki/index.php [L]