I have an inside source about this. The 10mph restriction was only imposed very late in the day (far too late to incorporate the impact into timetables for example) and was to mitigate an issue which only came to light very late into the installation.
Failure to mitigate this issue would have meant BASRE could not have been commissioned, or at least require severe loss of functionality at Bristol TM.
Basically, with back-to-back mid platform signals, the block joints are positioned directly underneath the signals. The block joint for one direction is the signal replacement joint for the other. So a train arriving across the mid-signal releases the track circuit in rear once the last axle has passed the replacement joint. Problem is, modern rolling stock have increasingly long nose cones, and it was realised that the last axle could clear the replacement joint and stop with the rear nose / couplers still occupying the section behind. The signalling system would not be able to detect this and could then release the East Gantry / West Gantry junction signal to a single yellow without the near-end platform actually being clear. This could be interpreted as a wrong-side failure.
The mitigation is therefore to impose the 10mph and warn drivers to approach the mid-platform signal at extreme caution as there is a slight possibility that another train could be standing foul without occupying the track circuit. Unfortunately speed signage can’t be made signal-aspect specific so it will apply to all trains regardless of movement. Interesting the comments about whether it applies for the whole length of the platform or not - I had heard it would be to the mid-platform signals only. Either way, this restriction is going to be exceptionally difficult to remove, without redesigning the interlocking or moving the mid-platform signals about 8m apart, which would allow the block joints to be positioned in a more appropriate position.
I must admit, I can’t help looking at this situation and just seeing engineering incompetence and loss of knowledge, although I’m sure this isn’t an entirely fair reflection on the people involved.