But "best clothes" as oppsed to "party clothes" can be used for all sorts of things, if they're adaptable enough.

Not in their situation, they can't: Nicola and Lawrie are going to parties in the Christmas season or wearing "that dreggy uniform dress" at school. The same would have been true of Ann and to a certain extent of Ginty. Rowan may well need a "good" outfit to meet with bank managers, as Karen no doubt needs one when she's at Oxford, and no doubt they have what they need.