Cascading is no longer that easy. For the ADL/BYD buses, there were 80 dual gun chargers (for up to 160 Buses) installed in Caledonia. This involved 3 11kV substations, and Scottish Power constructing a new 33/11kV substation within the bus depot (this is owned by SPEN so is not entirely dedicated to the bus depot). For the airport Electroliners a further 11 charging points have been installed of varying capacity, but 3 of them are 360kW chargers, and 2 others dual gun so allows up to 13 buses to be charged. AFAIK there are only 11 airport buses, and currently 5 repowers, so they may be working chargers harder. For reference BYD E200EVs for M3 I believe use AC chargers so they are separate and specific to these buses. Don't know how the two demonstrators are set-up, most likely standard CCS DC chargers.
The charger layouts and electrical supply were based on specific bus types (length and battery re-charge time), so moving buses around is more involved now.
That being said, for First to meet their targets, charging will be needed everywhere so cascading EVs is still going to happen.