”a. contacted me to let me know they were advertising a role whilst I was on maternity leave, especially one that contained 50% of my responsibilities (lets call this job X)”
Yes they should have done. Did you complain about that at the time or ask about applying?
”b. if their plan was to get rid of my old role, job Y, I should've been consulted and given option to be redeployed else were or given a few weeks pay to leave.”
Yes, they should have consulted you, or, if they didn’t feel it was reasonably practicable to return you to that role, tell you that and explain why. You say you were “encouraged” into a new role. Did the question of your old role not even come up? Did they not explain why they were “encouraging” this?
”c. My being pushed into an alternative role, call it job Z, I've been set up to fail as it isn't my skill set”
Presumably they would say you agreed to it, and if you felt you didn’t have the skills, this should have been discussed at the time. It would be incredibly unlikely for an organisation to deliberately set someone up to fail in a job and leave them in that role for months. If they wanted you to fail at it, I would expect there to have been robust performance management during that time, and for you to be on formal warnings by now.
”I've gone from having complete autonomy to now not being able to send an email without my manager proofreading it. He WhatsApps me during meeting telling to stop talking, say this or that. It feels very much like a demotion (my now manager was my peer previously), I do understand this is hard to prove though.”
That sounds like an issue with the manager which could happen in any job.
You may have legitimate grounds for complaint about some of this. But that doesn’t translate to any kind of legal claim. It’s too long ago- you have three months to bring a claim of pregnancy/maternity - related discrimination and it sounds like you are way past time, and didn’t even raise a grievance at the time let alone take anything further. The time to react/resist your redeployment was at the time it happened, not several months down the line.
You have a new job and can move on. That’s really positive, so focus on that.