Finish and foam
White postal boxes are a 125WK/T finish, including shell-and-slide versions with foam inside. Packaging Boxes lists them as qualifying for Royal Mail small parcels.
That does not raise the maximum. The outside limit is 450 x 350 x 160 mm. Brown single wall is the everyday board. White is the presentation.
With the other cartons
They can be ordered with tape and fill. In by 3pm, they leave that day. Stocked cartons can be returned within 14 days of purchase.