I'm surprised at the 'get a new windscreen on insurance' and 'a bargain for 100 quid' attitude. There are a few things to consider.
Your insurer is not interested in your car. They're in the business of indemnifying loss; they take your money and don't really like giving it back. When it comes to paying for your new windscreen, most - if not all - inscos will have some kind of an 'average invoice price' deal whereby the insco gets a cracking deal (pun intended) on glass replacement. That one AIP might even include repairing chips, cue those 'there is never a good time to have a chipped windscreen' adverts...
On the flip side, the 'nominated' windscreen company will be looking at some serious volume to make that AIP deal work, so they will no doubt have secured as close to exclusivity as possible (which is why you'll find that the insco won't be too keen to let you nominate your own preferred supplier). AIP deals work by sheer volume, and by bringing in cheaper glass into the equation. This cheaper glass is coming in from China, Poland, and even that once British brand Pilkington is Japanese, owned by Nippon Sheet Glass (and the windscreens they produce come from plants all over the world). The claim that aftermarket (copy) glass is as good as is not entirely true, and not even the manufacturers are making a definitive distinction between OEM and aftermarket glass. See
here for some examples of this. Paying your excess will rarely get you a like-for-like genuine LR windscreen; it costs too much.
Paying your excess may also open you to a risk of a poorly fitted windscreen by someone with little experience or with no regard for your car. It is alarming how many fitters/fitting companies are installing windscreens which are contaminated (from manufacturing processes) and the damage a lot of these guys are hiding underneath your shiny new windscreen will only make itself known when the warranty period lapses, or when the car is sold (windscreen guarantees are not transferable).
Aftermarket heated windscreens are functionality tested, but you'll only get a 12 month manufacturer warranty. Fitting guarantee will vary from firm to firm.