Ok, even if it is not related to the pregnancy, they still cannot sack you.
When does your annual year start and end? If you have taken too much annual leave from the last year, then that should have come up around The end of April (if you run from april to april), why they are bringing this up now is a question to ask.
If your sickness is preganancy related, then again, they cannot sack. Further more, your manager saying that they don't believe you is totally unacceptable and something that you should bring up in the meeting. Your sick not/fit note is a notice from your dr telling them that you are not fit to work and they need to heed this, even if it is a hindrance on the workplace.
If your illness is related to the pregnancy, then you Mat leave is automatically started from the 4th week of EDC.
You are allowed to take you Mat leave from the 15th week of EDC and you should take all your expected annual entitlement for the year before this. Even if yuo are on Mat leave you will be accruing holiday entitlment as you are considered to be working.
As other have expressed, you need to take someone in with you. Take notes, have questions ready to ask them. Ask them what they will do to facilitate you going into the work place if your dr feels that you are able to return to work, and when they are going to carry out a risk assessment for you (if they have not already done so.) and that this should be carried out in front of you so that you are aware of the adaptations they are going to make!