Want to display a maintenance page from the F5 using ifile + irule and no cache.

HTTP::header insert

in the HTTP_REQUEST event inserts the headers in the request before passing the request to the forwarding target (generally a pool member). Since you are not passing the request along, that's not quite what you want.

The HTTP::respond command allows you to insert headers directly on the response:

when HTTP_REQUEST {
    if { [active_members [LB::server pool]] < 1 } {
        HTTP::respond 200 content "[ifile get MY-HTML-IFILE]" noserver "Cache-Control" "no-store, no-cache"
    } 
}