Feel like work are trying to force me onto maternity leave early

[Slh122]

I posted on here last week regarding my footwear situation at work (under a different name I think)
When I first fell pregnant my pregnancy was being treated as a suspected ectopic and I had a few days off work. I let my manager know the situation. When I returned to work I decided to apply for a promotion that came up. My manager told me I wouldn't be considered for it as I was pregnant and in her words 'in 7 months time they'd only need to replace me so it's pointless'.
I went to HR about it and they decided I could apply and I was given the promotion.
Now I am 33 weeks pregnant and quite big. 2 weeks ago I was told I could wear my own maternity clothes to work (I work in a customer facing environment in fashion retail) and could wear my Uggs. Last week she told me I couldn't wear my Uggs anymore - fair enough, I went and bought some new black shoes that I could wear comfortably.
However yesterday I was suddenly told I had to wear my work uniform (dresses etc) again and that if I came to work in anything else I'd be sent home unpaid.
So today I have come in in one of my work dresses. It is very tight and doesn't fit me well anymore. My manager has said I am dressed inappropriately! I have been told to buy something else from work to wear - I have no uniform allowance left and was told I had to buy it myself. Even with my staff discount a dress from there is going to cost a considerable amount!
she then took me aside and said that yesterday she noticed I was 'standing around doing nothing' which is completely untrue. I am not lazy at work. She said they don't feel I can do my job properly anymore and have suggested I 'revise' my maternity leave start date.
Another girl at work has apparently been offered my position when I go on ML. I pointed out I would be returning but she told me her and work have just assumed I'm not coming back so now it looks like I don't even have a job to come back to.
What can I do?

Keeping a log of everything is a very good idea.
While it's fresh in your mind write down a description of the tellings off.
i.e. Date- Approximate time- What you were doing- What she said-etc....

In practice; Yes they can send you home for any reason.
It would be very likely to be found that it was a pregnancy related detriment to do so because of the policy.
Sending you home unpaid would definitely be a detriment.
All these incidents add up to harrassment.

If you can access e-mail on your phone e-mail yourself a note of what's happened as soon as you can. That date stamps it so you know exactly what was written when.

As HR are notoriously bad at dealing with things; if you like I could prepare a different version of the earlier grievance. Keeping in the main facts, but referencing which sections of the Equality Act 2010, the Maternity and Parental Leave etc. Regulations 1999 and the Employment Rights Act 1996 your employer has breached.

You know them, so do you think that receiving a letter that they should (perhaps with a little googling) be able to recognise as full of potential Employment Tribunal claims against their employer would be more likely to focus their minds and take action to deal with the issues.