Fair enough; there didn't seem to be obvious logic to, e.g., the line of 5 signals on exit from Portsmouth & Southsea are 450, 452, 64, 426 and 270 from left to right looking towards Fratton, with 66 and 272 as the high-level starting signals.
Please keep in mind that just like anything else on the railways, there are multiple different standards used at different times. Because you can’t have just one standard…
For example, Western Region had their own standard, while other regions had theirs…
And the then there is the current Network Rail standard (that I don’t like).
Prior to BR, the various different railway companies each had their own standards.
Also, mechanical signal boxes used a different system, based on the physical position of levers in the frame to make the mechanical interlocking practical.
On top of all that, alterations, additions, and recoveries, possibly followed by further changes, all could disrupt the standard numbering that was/had been in use.
Hence, why in some places, it’s hard to see a / the pattern…
I’m completely biased of course, but I did like the system used by BR Western Region.
Under this system, for controlled signals, for the up main line, the signals were given odd numbers, prefixed with the letter or letters for the signal box/panel, with the lowest being furthest away from the lowest mile post, and counting up (using odd numbers) towards the lowest milepost. So a up train would see X1, X3, X5, X7 etc.. (the X being the signal box/panel identification letters).
For controlled signals on the down main line, even numbers were used. But started counting from the lowest mile post and counted up towards the highest mile post. So a down train would see X2, X4, X6, X8 etc..
Other lines would use a similar number sequence, but would start at 101, 201, 301 etc. Freight lines, passing loops, exits from sidings or similar would use 401 etc..
Hence on the South Wales main line, B101 was on the Up Tunnel line at Pilning, with B401 nearby for the exit from the up freight loop.
At large stations, it was not practical to use different sequences for all the parallel lines. So here, signals were arranged to read left to right, increasing in turn. Hence at Bristol Temple Meads, the down direction signals at the western end of the platforms (and through lines) went B30 (platform 15, originally platform 13), B32 (down through), B34 (platform 12), B36 (platform 10), B38 (platform 8), B40 (platform 6), B42 (up through), B44 (platform 4) etc…
Automatic signals were different. They used letters derived from the line name and a number derived from the mileage. So on the up main in the area of say 154 mile post, a signal would be called UM154. Similarly the nearby down signal would be DM154.
It definitely made it easier to remember than the current Network Rail standard, which in my area has all signals prefixed with the same letters followed by a four digit number. Because the reversible or adjacent lines are included in the number sequence, a driver will not always see consecutive signal numbers on the line that they are on.
