The switch from series to parallel was part of the normal acceleration sequence. I found the following paragraph on the National Preservation forum from 'twofnine' that indicates the procedure (my emphasis):
We honestly believe that constant use of a 400 on that branch, would burn the resistances out eventually, and eventually burn the motors out. The CEPs, CIGs, and the VEPs have a Normal and Series switch in the Driving Cab. And that is the only time the switch was ever used on the Folkestone Branch. It gave us full series, and the speed up from the harbour was not much more than 10 mph, and upon arrival at the east; the resistances were very hot. They had of course cooled down after departure on route to Victoria getting into Weak Field.