Fuji, I'm quite open to criticism where valid. If you were talking of LARS units willfully refusing to provide a service outside CAS then valid point, but you're not.
Resources are finite and for non-LARS approach units providing services outside CAS to traffic that is not inbound or outbound is way down there at the bottom of the pecking order. Unfortunate but that's the way it is.
Services will be provided if capacity allows but won't be when/if it doesn't. If you want more than that then resources are going to have to be found and paid for or something else is going to need to happen. I've already alluded to the latter...
IO540, the MATS Pt 1, general instructions relating to all ATC units, is in the public domain and can be downloaded from the CAA web site.
The MATS Pt 2, which all units also have, is the book of local instructions/procedures relating to that particualr unit. In the case of NATS units all the Pt 2s are "NATS Commercial in Confidence" so one would be taking a bit of a risk if going in to them too deeply in a forum such as this.