C4 envelopes are designed for A4 to be inserted unfolded or A3 to be inserted folded in half. They are available in white or brown (aka manilla). Board back envelopes are also available with DO NOT BEND printed on them for sending items that you do not want to bend like photographs or certificates.
Gusset envelopes are designed for sending bulky A4 items like catalogues or brochures. The envelope expands to avoid the paper from splitting.
Octopus offer free delivery in and around Manchester & Cheshire incl Salford Stockport Altricham & Macclesfield and next day delivery nationwide including London, Birmingham & Liverpool.
The brands that Octopus stock are Plus Fabric, Basildon Bond, New Guardian, Survivor and Jiffy. If there is an envelope available, Octopus can supply it.