When your insurer denies your request for a genuine ‘OE’ windscreen replacement, and insists that you might have to pay the difference between what they are prepared to pay for and what you want, ask them why they appear to be okay about paying for dealer parts if the car is in for accident repairs.
An example:
A chap had his windscreen replaced after it cracked. He followed his insurer’s instructions and their nominated repairer arrived to fulfill their obligation. The car is registered 2017, so fairly new and very much under warranty. The make and model is irrelevant; academic, as a claim is a claim regardless of the cost. This is the windscreen they fitted:

As you can see, it’s a Shatterprufe windscreen (a South African company) which is merely a copy of the original (and not manufactured in accordance with the blueprint which the car manufacturer owns). It is much cheaper than the original (a fraction of it, in fact) and it allows the nominated repairer to service their agreement by volume of work on the basis of exclusivity. The process, from the car owner’s point of view, was easy and as straight forward as he could hope for; he paid his excess and everyone lived happily ever after.
Some time after the event, the same chap in the same car had an episode with another driver on the road, and the car had to be taken into a crash repair centre. Just like when his windscreen was replaced, he was instructed to take the car to his insurance company’s nominated repairer. Again, all fairly easy and the car was returned to him after the repair work was completed. There was however, something a bit different in this process. All replacement parts were original equipment, i.e., genuine ‘main dealer’ parts. They were the same as what the car manufacturer used when the car was assembled. One of those replacement parts was the windscreen. This is what the same insurance company agreed to pay for and authorised the fitting of:

Glass cover and accident insurance are two components of the same indemnity, so why the disparity? Why apply dissimilar conditions to the same product which results in the use of premium products in one scenario and cheaper, inferior products in the other?
This is a stranglehold you are placed in by your insurer who do not make this clear before policy inception. Try removing glass cover from the proposal: “the computer says no”. Try to ascertain what will happen in the event of a claim scenario and it’s not really that clear. In fact, it’s confusing but you have to find where it states what the outcome would be in the event of a claim, and when you do – if you do – it’s difficult to understand.
The aim of Insurance Conduct of Business Sourcebook (ICOBS) is to ensure that customers are treated fairly. One section in particular is not being observed properly:
“A firm must take reasonable steps to ensure a customer is given appropriate information about a policy in good time and in a comprehensible form so that the customer can make an informed decision about the arrangements proposed”.
In failing to define the outcome of initiating a claim by not stating that they will steer policyholders into using their nominated repairer (who in turn will use cheap and inferior parts) they are not presenting an important fact – or salient point – of the proposal. This should take place before policy inception, or, before you click on the ‘I accept the terms and conditions’ button. Conversely, the ‘proceed to payment’ button is harder to miss.
Funny that.