The only Rook rifle I have with that action is a John Wilkes 300.
Even Wal Winfer and Tom Rowe, in Vol. 8 of their British Single Shot Rifles book series, admit that they don't know the origins of these actions but mention that they could have been from Wilkes.
The barrel on mine is from Adams & Tate.

If there is no obvious method of detaching the forend, it is very likely a simple snap on version.
I know some of them take an inordinate amount of ooomph to get off and it is a bit disconcerting to put that much force on them.
Mine has a push button on the tip of the forend fortunately.

Here's my example: