My DD is 3 so she's pretty good about being able to put on the gear herself. As most UK nurseries are, there is a lot of open free play where the kids can choose if they want to be indoors or out, so she will be out in all sorts of weather. Maybe get both is good and let her practice at home, like PP said. Then see which is better for her. I didn't think of the potty issue, but that's a good point. She is totally capable of handling that herself, but a one piece may be harder. Her nursery has a full garden with actual vegetable garden, hill, path for cars/bikes, playground, etc. and the teachers said kids dig in the dirt, etc all the time (they even just did a "lesson" on worms) so I am sure she'll be out there a lot and in mud and all. I have a meeting with them in Dec to go over everything starting in January term, so they probably have the best advice, experience.
Good point on general clothes at school - won't send her in her nice stuff