Unless you have dedicated and known utility corridors (eg, concrete troughing), we are taught (even compelled by HSE guidance) to assume that cables runs will be haphazard in routing and most certainly not as indicated on historic plans (which are often schematic rather than precisely dimensioned records).
To get round this, there are a suite of tools & processes available. The most user friendly is ground penetrating radar type surveys which use several cable locating methods and which can give a 90 to 95% accurate 3D survey of an area and any cables, pipes or other buried underneath. These give you a pretty good steer when doing your design and help inform the contractor of working methods, risk and time allowances that will be needed. These surveys are then usually double checked with CAT surveys and hand dug trial pits during the initial stages of construction to physically prove there is no clash. Any works within set distance from known or suspected cables must be hand dug anyway. These safe distances vary depending on type of cable \ pipe, owner of the pipe and what it is you are doing.
For piling works, the exclusion zones to fibre optic cables, mains electric cables, gas & water mains are several metres. I get the impression that NR doesn't (or didn't) have any such rules on piling next to its signal or power cables with the results clearly evident in the Thames valley.
Whilst doing these surveys in advance of designs on a live railway is going to be slow and expensive, it would still have been quicker and cheaper than all of the broken cables, lost service and delay minutes incurred so far not to mention the political and commercial consequences currently being played out on the GWML by having late running designs, late running construction and a fleet of new trains with no where to run.