iRule to redirect URL based on source address

You could use a simple iRule like this

when HTTP_REQUEST {
    if {([HTTP::uri] equals "/admin") || ([HTTP::uri] equals "/admin2")} {
        if {!([IP::client_addr] equals "10.0.0.1")} {
            HTTP::redirect https://mywebsite.co.uk
        }
    }
}

You could also use a datagroup to store your IP addresses if you plan on using more than one:

when HTTP_REQUEST {
    if {([HTTP::uri] equals "/admin") || ([HTTP::uri] equals "/admin2")} {
        if {!([class match [IP::client_addr] equals "my_datagroup"])} {
            HTTP::redirect https://mywebsite.co.uk
        }
    }
}

You can take this a step further and add the URIs to a datagroup too:

 when HTTP_REQUEST {
    if {[class match [HTTP::uri] equals "my_uri_datagroup"]} {
        if {!([class match [IP::client_addr] equals "my_ip_datagroup"])} {
            HTTP::redirect https://mywebsite.co.uk
        }
    }
}

You need a data-group existing that contains a list of allowed IPs or subnets for this iRule to work.

when HTTP_REQUEST {
  if { [string tolower [HTTP::path]] starts_with "/admin" and !([class match [IP::client_addr] equals data-group_of_IPs_and/or_subnets ]) } {
    HTTP::redirect "https://[HTTP::host]/"
    }
}

Though it is possible to accomplish this via an iRule it is more computationally efficient to configure an ltm traffic policy:

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/local-traffic-policies-getting-started-12-1-0/1.html

Configure your match statements (i.e when the uri is/contains /admin2) then use the redirect action to point things back at the main URL when the match conditions are fulfilled.