The rulebook describes time interval signalling as the default method of signalling, and gives the above instructions for specific lines (which can be inferred were at the time single track). The inference is, therefore, that this is the method of signalling adopted for single lines when time interval signalling was used on double track sections.
Remember, the same type of signal is needed for junctions. Earlier on in the rule book, we have this about junctions:
"JUNCTION SIGNALS
...
"42. The Semaphore Signals must always be kept at Danger, except when required to allow an Engine OR train to pass, the Caution Signal must then be shewn; and in the event of Trains from two or more lines approaching a Junction at the same time, the Danger Signal must be kept on, until each Train comes to a stand, when the Branch Train must take precedence in passing the Junction points, except when specially ordered to the contrary."
However, the instructions for the Halifax and Ipswitch dock junctions (under "instructions for switchmen") are intriguing:
"59. The Telegraphic Signal communications between these points is worked as follows, and which must be strictly adhered to until further orders:- A single ring of the bell from one Junction to the other, if replied to by a single ring signifies "All Right," Train to proceed; if replied to by a double ring it signifies "Caution," Train to proceed very slowly; but if replied to by three rings, it signifies that hte Line is blocked and that all Trains must be stopped. In like manner, if one Signalman sends a signal to the other of one, two, or three rings, as the case may require, it signifies "All Right," "Caution," or "Danger," and it must be replied to by the same number of rings, to let it be understood that hte Signal is received; and when the Danger Signal of the rings is given, and until counermanded by the All Right Signal of a single ring, nothing must be allowed to pass between the Junctions. Also when no reply is given to a Signal, it must be considered as dangerous, and no Train be allowed to proceed until the All Right Signal is received; or in the event of no Signal being received after a lapse of five minutes, the Trains may then be allowed to go very slowly indeed..."
This does strike me as unclear as to the extent to which time intervals were actually used at junction locations. I could replace a full block reservation with a directional reservation for time interval with telegraph signals, allowing trains to proceed in the same direction, but it is unclear how this actually worked.