Replacing the electricity supply to a 7-storey 1930's block of 126 flats

Why is it being replaced ? Have there been overload issues ?  The biggest problems with older flats can relate to the building network - that is the cables not within the flats and therefore not the responsibility of the flat owners, and yet that are needed to supply them.

   https://www.ssen.co.uk/globalassets/building-network-operator-bno/bno-customer-guide-v1.0-1.pdf

Quite often this network includes cables prior to metering, that need to be secured against theft. At the same time, not to pass through any flat except the one it supplies.. If there is a central metering panel and wall of isolators one per flat, or one such panel metering per floor, it gets in some ways easier but then tenants and meter readers need access... Equally a lot of sub-mains requiring anti-tamper security because metering is in the flats rapidly becomes a pain unless designed in from the start. Then there is safe isolation if one flat needs a new meter, you dont want the whole building off, so red (unfused) and grey (fused) links in service heads or Ryefield or Lucy style cabinets come into play

The idea is that there is a 'building network operator' responsible, but in practice that is often whichever hassled contractor the freeholder can arrange at short notice when there is a problem, so it needs to be self explanatory and clearly labelled.

The building network needs to be to BS7671 as well- which in a lot of older buildings it isn't,  being perhaps concentric cables or hairy string in un-earthed conduits put in before DNOs existed, let alone BNOs.

Normally wiring in flats is the leaseholder's problem, and they must make it safe for any tenant if they sub-let and the BN and common areas are the freeholders /common-holders  but it will depend on the contractual detail of ownership of the flats.

Mike

As the flats are existing, it should be possible to fit a data logger to the existing supply in order to obtain maximum likely load. I would allow for say 10% load growth. If the present supply is 315 amps, but routinely overloaded to say 350 amps, then consider a 400 amp supply for future needs.

If say 8 flats are empty when the data logging is done, then an extra allowance should be made for future 100% occupancy.

If gas or oil heating is used at present, then a much larger allowance for load growth should be considered in order to allow for future electric heating.

What about charging electric vehicles ? are EV chargers to be provided, if so allow 32 amps for each charger, 126 flats implies 42 on each phase. If each flat will have a 32 amp charger, then the total loading will be much less than 32 times 42. They wont all be used together ! If however a limited number of EV chargers are to be installed, then the loading per charger will be much higher, as all or almost all of these chargers are likely to be used at the same time.

Peter, why are you asking please?I think that the key to this is who is responsible for what. On the one hand, there might be a freeholder who is responsible for getting the supply from the electricity company (DNO) to each flat; or on the other hand, the leaseholders are jointly responsible.

Does photovoltaic energy exist, or might it do so in the foreseeable future?

It might be worth considering future plans for electric heating/cooking (heat pumps/induction hobs), the possibility of solar PV and battery storage, and, if relevant (if there are parking spaces associated with the premises) electric vehicle charging.

However, no-one has a crystal ball, so, as others have said, why is the supply being replaced, and overall what do the residents think they might want in the short/medium term, and what do residents think about the future?

Really interesting, perhaps worthy of a longer term case study to assist in informing the notion of maximum demand for similar situations as we move towards a more electrified future. 
It might be that irrespective of any accurate MD assessment, the existing DNO capacity might be the big issue. Any necessary change on the HV side could result in eye watering costs.