If you are a low mileage user, you are better off buying a high mileage 100,000 diesel Honda/Toyota or similar, it work for us. When we decided to change to 12/18 month old low mileage vehicle but still being low mileage users the depreciation was a big hit so we changed back to Motability.
Diesels aren’t great for low mileage users though, as you can get problems with the diesel particulate filters. As you say, 12-18 month old cars are still going to incur a lot of depreciation, so won’t necessarily be a good buy either.
I think that the ‘sweet spot’ for those of us who would consider going down the private route is buying a 3 or 4 year old car. They have taken the vast majority of the depreciation hit by that stage (most will be half the new price or less) yet are still pretty much like new and will have a good 10-15 years left in them (not that I’d keep them anywhere near that long). This approach also opens up options to get cars that are a few classes up from what we are ‘allowed’ on the scheme, yet whole life costs will most likely be less than running a more modest new car through Motability.