I'm not sure if its changed round here, but I think the usual thing used to be that the SP ones were often changed live, whereas the TP ones were made dead. I know of one situation where a TP one was to be changed in a commerial premises, it was all planned for a saturday morning, pabvement outside was dug up to expose the cable, then the whole job had to be called off, the servcie cable ran too close to the building foundations and there wasn't enough room to be able to joint it. I don't know what the eventual outcome was, but I expect it was a few months delay and a bigger hole with the cable jointed further back and possibly a new servciecable into the building from that point onwards.
Northern power grid can be a bit of a pain to deal with sometimes, and I never can work out quite where the NPG / WPD (former EMEB area) border is, there are certainly areas with a lincolnshire postcode and address that are within old YEB area!