Is this a nursery that takes babies and up? If yes then my post will be relevant. If it's a nursery that just does the year before reception then I'm not sure if the rules are the same?
Have a meeting with the nursery manager. Say no to potty training right now, and explain why. Query any 'rules' carefully.
I had the same issue (in a full time nursery from babies up to school age), and it turned out that the more junior staff had taken it on themselves to make up rules about DS not being allowed into the next room/class unless he was fully potty trained (at 3yrs). They artificially held him back in a room he hated and was miserable in, almost as a punishment for not being potty trained. DS was being told all the time that he wasn't doing well enough, not a big boy and just a baby, unlike child x (basically any other child), who was so grown up and was going to the big class where all the big boys go, but he had to stay with the babies etc. It became cruel and not at all motivating for poor DS who just internalised all the negative crap they were saying to him (
)
Turns out the rule wasn't even true, and in fact, I'm not sure they're allowed to make that kind of rule (?).
I understand that they have issues with staffing numbers and having children needing 1to1 time nappy changing means their ratios get more tricky, but a good nursery needs to come up with a solution which doesn't involve excluding non potty trained children or forcing it on them regardless of their ability to do it or not.
In our case, the manager agreed that forcing potty training on a child when he wasn't ready was going to do more harm than good. And holding him back behind all his peers & friends, on the basis of potty training was not appropriate.
DS trained within two days when he was ready, about 8 months later. I suspect he would have been able to do it earlier had he not been so upset by the cack handed way the nursery assistants tried to do it. It took a while for him to get over it, poor thing.