Congratulations on your pregnancy!
Completely agree you should ask for everything in writing. Retain everything they write to you.
You have worked there for two years so you are protected from unfair dismissal. That means you have to fired for a good reason.
The law treats you as being in a protected class because you are pregnant. If you are fired because you are pregnant it is treated as being automatically unfair in law and if you bring the claim to a tribunal, it can order your employer to pay additional compensation (calculated based on your years of employment I think), or you might be able to get your job back but I am not sure under what circumstances the tribunal can order that. Monitoring your productivity because you are pregnant absolutely sounds discriminatory.
This sounds really hard. You’re right to be careful.
I would google unfair dismissal maternity lawyers and call one that offers free initial advice. They can give you information so you get a basic sense of the lay of the land. Don’t feel pressured into instructing them, good lawyers won’t press you to right away.
Also be aware that you have to bring an unfair dismissal claim within six weeks of being terminated so you don’t have much time.
Litigation is also stressful so you may want to consider asking the case to be stayed until after the birth. Not that things will be easy with a baby, but it’s something to be aware of.
If you have a Citizen’s Advice Bureau, you can go to them for advice. You can also google pro bono lawyers in your area to see if there is a free advice clinic.
So sorry your employers are showing such a bad side!