Firstly, Gateline staff are not on minimum wage.
Secondly, the way shifts work means that for 2 staff on duty to cover a station every day will need about 8 - 10 people on the roster, to cover things like leave, training, absence, etc., and that’s before we consider what happens with breaks.
Third, the very nature of shift work leads to enhanced pay for some shifts, whether it be Sundays, Bank Holidays, overtime, rest days, nights, whatever.
Fourth, with every employee comes a cost of supervision (ie additional management), training costs, and other back office costs eg HR, Finance, and all the rest of it.
Can't speak for SE as my experience was with another operator but we were paying just about minimum wage with no shift enchantments of any kind.
The number of people recruited for each station was closer to 6 full time equivalent with any gaps covered by overtime or agency staff, so no spares in the same way you might have with drivers.
Training was fairly minimal at recruitment with virtually nothing on going. From a management perspective we were expected to just get on with it, no additional resources provided despite a fairly substantial increase in headcount.