What the title plan actually shows
A general boundary. The red line on a registered title plan shows roughly where the boundary lies; it does not fix its exact position, and it is drawn at a scale where the thickness of the line covers a real-world distance.
Some titles carry T-marks indicating who is responsible for maintaining a boundary, and where they exist they are useful. Most do not. An unmarked boundary tells you nothing about ownership or responsibility.
A determined boundary — an exact line fixed by application to the Land Registry — is rare, and if one exists on your title it is worth knowing about, because it is the one answer that is not open to argument.