JLR dont actually build cars for stock as such, they release to each sales site a number of build slots in 4 week blocks roughly 2 months before build, this will show how many each week and which engines will be built, dealer will then match up his confirmed orders against those build slots, any un-sold slots will have to be specced for build, the dealer will use his previous knowledge to spec those cars and add any extras that he thinks will sell the cars, Disco's being considered a towing car will probably be specced with a tow bar as he'll know alot of customers will most likely want one, although he probably wouldn't order a "stock car" with a P4 engine and tow bar - these effectively become what you call "stock cars", they actually are allocated to a particular dealer. If you wander into a dealer,depending how proactive he is, he will try and find a car in the system which shows as an un-sold order, if he can match up he will have to negotiate with the owning dealer to obtain car for you, this will probably result in you getting a vehicle with some additional extras that you may not have wanted, but are prepared to accept rather than wait for a specific custom build.
This is what happened with my D5, I suspect I'm not what most sales people want as I walked in knowing exactly what trim spec, engine, required extras and a preferred shortlist of colours, dealer went into system and returned with 2 possible matches, the one I chose has front foglamps, that I wouldnt have specced, and another one with an additional £1000 of extras, all of which I didnt want, he then had to negotiate with the original ordering dealer to get it released for me.
24MY D300 Dynamic SE, Fuji White, Deployable Towbar, full size spare.
Gone 20MY SDV6 Landmark, Indus Silver, Deployable Towbar, 360 Cameras