Several thoughts, not all relevant directly to the UK, but worth discussing with your employer:
1. Some employers make bad financial decisions. It is that simple. When you are the decision-maker, you can change those decisions. Otherwise, you need to determine whether you can work under the bad policy or not.
2. Many businesses use fully refundable tickets when they are billing to a client. It is simply not feasible (or the limits of your employer's accounting system may make it not feasible) to hold non-cash balances on clients' behalf. Without some time with your employer's CFO, it's impossible to tell.
3. Some employers are sticky about their rules. Others are not. Many employers now permit the use of a corporate card to pay for some personal expenses or for the use of a corporate and personal card. In the first instance here, you would pay for WTP on the corporate card and then pay the employer the difference immediately. However, note that the UG would have to be to fully flex or refundable if that is what the employer wants.
4. Where you have a mess is the instance where dirt cheap non-flex WTP is cheaper than fully flex WT. In that case, you have a taxable income problem (as does your employer).
All of this bespeaks a relatively simple approach. As what you propose may be forbidden or the failure to disclose it may be viewed as untrustworthy, I would discuss what you propose to do with the employer. Quite clearly you would agree to pick up any change fees or losses from any situation in where a trip is cancelled or changed.