Also to bear in mind signals only read to the next available Main Aspect signal. In a complex area like Preston, the signals are close together to allow the driver to read a series of 'feathers' so the signal you've highlighted doesn't actually give authority all the way through the station - there's another set of signals protecting the platforms, each with their own Indicators.
this link may give you a better idea of the signalling layout at Preston
See trains move in real-time on our live map of Leyland to Lancaster
www.opentraintimes.com
Specifically at this signal, some lines are signalled bi-directionally, but not all. Immediately south of Preston you have 2 goods lines, 2 slow lines and 2 fast lines. the Fast and Slows are bi-di between Ribble Jn and preston but the Goods are not
At the signal in the screenshot (PN76), the train is on the Down Fast. Main aspect only for Platforms 3/4/5, No. 1 JI reads to the Up Slow (platform 2), No. 2 to the Down Slow (Platform 1), and No 3. to the Down Goods. In between the Down Slow and Down Goods is the Up Goods, which is not signalled for northbound moves, hence there is no JI reading to that line. No 4 Jn indicator reads to the Up Fast, where a further signal will indicate which platform (4/5/6/old 7 now UG Loop) the train is routed to. This is why there's only one JI on the right had side - it only shows the route to the next signal.
For reference the adjacent signal in the photo PN75 reads MA: Dn Slow, No 1 JI: Dn Gds, No 4: Up Slow, No 5. Dn Fast, No. 6 Up Fast.