I've shot 3 perf, but never did a side-by side cost comparison. As a general rule, it will be similar in cost, but you'll have to sit down with the producer and go over all of the numbers. I also don't know the numbers for the UK. 2- perf will be the cheapest film option there is as it's 1/2 the stock so also 1/2 the development for the same amount of time as compared with 4-perf, and, generally you can get a better deal on a film camera rental, especially if they are not in demand.
Alexa won't matter in terms of the horse/water/dusk thing in the least really. In fact it may slow you down if you have to set up a video village and all the crap for it. In essence you have x time to capture the scene no matter what format you shoot on, and then the light is gone.
I will say, if you are the director, and you want to shoot film, then you need to sit down with your producer and inform them that you want to shoot film and you want them to do all they can to make this happen.
It may work, it may not work.
You also should be calling around labs and rental houses on your own to get your own numbers to go over with the producer.
Now, in terms of digital cameras-- the only one I would trust is an Alexa. But then again, dealing with all that data is neither easy nor cheap. Are they factoring in the costs for a DIT and LTO tape backups for your "free" footage?
Both films will have to hit a color correction/di stage the only difference really being you need to scan the film negative after development. But again, you'd have to get numbers from your post house as there really isn't "x" cost published. I've gotten some great deals and some not to great deals.