Can these large sheets of 2.5mm thick Whiterock panels be bent into a corner shape without the expensive former, it can it be done with a hot air gun on a bench?
I've never used it but if it's PVCu based then in theory - yes.
Problem lies in over-heating. if it reacts like other plastic sheet materials, the point at which it can be formed is very close to the point at which it melts.
Heating a long strip with a heat gun won't be possible. I think you'll need a hot bar former unless it's only small pieces you want to bend.