Yeah I'm with you Derfred, still not convinced it would be illegal (if it's even happening) and statements like "I'm right but can't be bothered proving it" don't cut it for me.
Especially as we are not talking about an employer refusing an employee a transfer/promotion but simply delaying said transfer/promotion to a point in time that would not unduly damage the business's operation. I'd be stunned if the law would force an employer to take that imposition because an employee didn't want to have to wait.