I would also weigh in on the "keep the bath" side. Getting rid of it will definitely hurt your resale, and although you say you don't see yourself moving in the medium term, when you do you won't want the hassle of reinstalling a bath at that stage. I bought a house and redid the bathroom and I replaced the bath with a new bath, even though I haven't had a bath since I was a kid and don't forsee having one any time soon. But it's really no extra hassle to have it, it's nice to have the option (for myself or a housemate or guest) and I also do believe it'll be worth if I ever go to sell.
But you say that you've never been happy with the layout of your bathroom, so if you're spending money maybe there's a way to improve the layout without losing the bath. For example, you could get one of those baths that has a "bubble" at the shower end so you'd have more room when you shower.