Of the large volume UK door manufacturers, both Jeld-Wen and Premdor have expanded their range to produce better quality doors in oak (or part-solid part-oak-laminate). These will be available form virtually every builders merchant and you will be able to negotiate significant discounts if you are purchasing several. Wickes, as far as I know won't volume discount unless you are registered with them as a trader.
So if you want one or two, go to Wickes - otherwise try a builders merchant.
It is very probable that Wickes source their doors from one of these two suppliers anyway - and just brand them as Wickes.