irule to rewrite header but respond with 200

The iRule you have does not generate any type of response itself - it's just sending the user to the pool with a certain HTTP host header. I expect that the pool members are sending the 30x responses, so what do the pool members to see as/in the HTTP request?

I'm not sure what you are trying to accomplish. Redirects (301, 302, 307) are standard responses (per RFC 2616) that tell the client to retry the same request but at a different location.

A 200 (OK) would normally carry a payload (e.g. HTML, image data). It is not intended to instruct the client to treat it like a redirect.

If you're looking to manipulate the response you can use:

HTTP::response 200 content "your content here" header X-Custom-Header "custom header value"

Can you explain what you are looking for? Perhaps you are intending to rewrite the request (i.e. keep the same URL in the address bar of the browser but have the BIG-IP send a different payload/page? In that case you'll want to use

HTTP::uri

So this is the requirement from the customer

1. There are currently various "test" domains which are live, such as test.co.uk, test.es, test.com, and so on
2. A set of pages have been created using an application. This application is currently deployed on IP=1.1.1.1
3. This application’s pages will be available as www.ultimatetest.com/www.test.com/p/Pages (with each test domain having its own sub-directory)
4. For this to work successfully, we need the following:
5. ultimatetest.com DNS to point to 1.1.1.1 (where application resides)
6. URL handling rule: All requests to www.test.xx/p/$1 to go to www.ultimatetest.com/www.test.es/p/$1
7. Once this rule is setup, a request for a page www.test.com/p/xxxxx/xxxx will result in a 200 response for the user. This will work using the URL handling rule sending requests ahead without generating a response to the user and the 200 response coming from ultimatetest.com

maybe 5-6 domains but shouldn't change too often

This should be a good starting point

when HTTP_REQUEST {
  if { [HTTP::host] contains "www.test." and [HTTP::uri] starts_with "/p/" } {
    HTTP::uri [HTTP::host][HTTP::uri]
    HTTP::header replace Host "www.ultimatetest.com"
  }
}

https://devcentral.f5.com/wiki/iRules.HTTP__uri.ashx

https://devcentral.f5.com/wiki/iRules.HTTP__header.ashx