I think the OP's issue is that JL only stock up to size 16 - both in store and online- for a particular company, whereas that company do make the coat in a size 18, but if she goes directly to that company, she can't use her JL vouchers. Unfortunately, JL often do this with some ranges at both ends, I've found at the other end having small size 3 feet (or 2.5 in some shops!) that while some companies make certain shoes in sizes 2 and 3, in the consession in JL and other department stores, they only start them at 4. It is annoying when you have gift vouchers for a department store so can't go direct to the main shop of the brand.
The downside of a shop like JL that stock such a wide range of products, is they just don't have the capacity to stock every option for each range, for shops with a smaller range of products, they can stock every option within that product range.
They do need to think about their space vs the chances of them selling something, I recognise there just isn't a large number of people with child-sized feet like mine who want adult style shoes, so there's not the demand to justify stocking sizes 2/3 in all ranges, which could mean they are using up storage space so they can't offer a whole extra range to the 'normal' size feet woman.
In ranges aimed at teenagers, I'm more likely to find 2s and 3s as a lot haven't stopped growing yet so there's demand. If the coat you want is part of a range that's aimed at younger woman, there's less demand in the larger sizes, ranges aimed at older woman rarely stock the smaller dress sizes like 4 and 6.
But OP, I really wouldn't assume you'll be a size 18 still by Christmas if you have just had your first DC.