Gripping a 45mm / 1.75" metal disc to allow replacement of corroded toilet lid fixtures
That plastic nut is not going to unscrew off that corroded bolt. I don't have a Dremel rotary tool so my first thought would be to fracture the plastic nut with heavy pliers so that it could be torn off. I would try what is called here "channel locks" but has a different name in the UK. I have some very large channel locks that would grip the plastic nut with the jaws of the pliers parallel to the bolt. I would try to get the jaws a little off the centre line of the bolt. There is a risk of breaking the porcelain in this approach that one must be willing to accept.

Another approach would be to use a hacksaw blade. Grip the blade with a heavy glove or a rag and saw into the plastic nut parallel to the axis of the bolt. Saw all the way to the bolt and then tear off the plastic nut with some pliers.