Consumer unit location
There is/was nothing in the Wiring Regulations to require or prohibit such an arrangement.

It is/was not unusual to have electricity meters in communal cupboards, especially in larger blocks where a concierge could open them for the Electricity Board meter reader without the residents having to be disturbed.

Many 1970s flats had various types of electric heating, such as storage heaters, underfloor heating, panel heating, and water heating. Separate tariffs for ordinary use, water heating, and space heating, were common. This would need a 3-rate meter and a timeswicth or radio teleswitch to change the meter rate and turn on/off various circuits, and 3 main cables between the meter and supplier's timeswitch or teleswitch equipment in the cupboard to the consumer unit(s) inside the flat. Heavy main cables are expensive. To allow for the possibility of heating being changed from storage to panel heaters, etc, all the main cables would have to be sized to take the full load.

Also, meter 'tails' or submains longer than 3 metres are supposed to be fused near the meter, as the supplier's cutout cannot be relied upon to clear a fault quickly enough. That means a switchfuse next to the meter (more expense, and takes up space in the meter cupboard). 

Therefore, putting the consumer unit(s) next to the meter in the cupboard, and running a few extra metres of circuit cables to the flats may have been considered the cheaper and more practical option at the time. 

An example of a multi-rate consumer unit from the late 1960s is shown here

https://www.flameport.com/electric_museum/wylex_fuse_boxes/wylex_multi_rate_metal_fusebox.cs4

It will be noted the fuses are grouped into separate sections, each with their own main switch: lighting and power, heating, and water heating. This could have been supplied by 3 separate mains cables and a 3-rate meter.

Most of these multi-meter multi-rate tariffs and heating setups are now obsolete.