The X signals by T5 are related to the adjacent evacuation shaft. Yes, the last auto before a controlled area is two, floodgate X signals is three, surface stock detector X signals is four and data collection signals X855 between Manor House and Turnpike Lane and X883 between Wood Green and Bounds Green. These data collection signals are to do with the signalling control at the east end of the line and the need to stop trains entering the section if the signalling system which was then run on an old computer failed, or so I'm told!
Ah - well if you're counting the various different reasons for non-prefixed X signals then I would suggest there's 6 'varieties' on the Picc, because (as I mentioned) X402a/b/c are Xs for a different reason to X400 (and X399).
TBH, the whole signalling arrangement on the WB approach to T5 is unusual with 4 "X" signals plus a draw up signal plus an illuminated shunt that is suppressed when not showing a proceed aspect.
Shunt signals associated with a running signal (even if that running signal is a Fixed Red) should always be suppressed if they have not been cleared for the shunt move. It's the same on the Central ljne and a few other places which use fibre optics.
As for the four X signals in a row, try the C&H around Paddington' all of which are associated with different things: TRX123, X125, X127, X129, OPX131, OPX133 (EB) The Westbound is similar but one 'A' signal in the island: X134, X130, A128, TRX126.
TRX123, X125 and TRX126 relate to the detection of the Royal Oak crossover X127, X129 and X130 relate to the detection of the Paddington crossover. OPX131 has a hold facility controlled by Edgware Road OPX133 is a 'traditional' X signal approaching a controlled area. X134 is associated with a Track Circuit Interrupter associated with Network Rail platform 14 at Paddington.
TRX123, X125 and TRX126 relate to the detection of the Royal Oak crossover
X125 has only recently become X125, up to a couple of months ago it was A125 but the instruction to drivers was to treat it as an 'X' signal until someone got around to changing the signal plate on it!