If no-one would buy the club for £125m, then it isn't worth £125m. Like anything else, the club is worth exactly what the market will pay for it. That said, I still don't see how being in the Championship would cause the club to be more attractive to a buyer or induce him to pay more for it.
If you look at it like buying shares in a company, you wouldn't buy them at top dollar, WBA's stock is probably around 90% of it's potential, to increase that, would take a degree of risk, so if a share is worth say £100, why would you risk that to make £10?
If you buy at a reduced price, then the earning potential is much greater.