Ok firstly as they are your employers, they are entitled and indeed probably required to make contact to check on your welfare.
As this seems to be an official visit, I believe you have the right to ask for someone else to be there with you.
If you are in the union I would definitely ask for someone from the union to be there and take notes and if they are unable to make that date and time then you could ask for the meeting to be moved to a time that they can make it.
Failing that is there anyone else that can come along, a close friend or someone you trust. I don't think they need to be from work. Alternatively you could request someone from HR to be there.
I really think from all you have said that a third person there is vital as you don't have a good relationship and you do want to make sure all the vital points are covered.
Personally I would go for the home visit rather than in the office, you are on home territory and as someone who has had to do home visits, it does make the whole thing more personal and less work focused than it would be in the office.
Prior to the meeting have a think about what points you want to get across such as :
a) Level of sickness - don't be shy to be detailed about this, maybe a list of how many times per day you are sick or something similar
b) What the GP says - Do they think it will ease of or are you likely to have this for the whole pregnancy, you poor thing if so
c) Your keeness to have an OH referral for both you and your managers best interests
If it were me I'd make a list of what I want to cover.
Also make sure that you are demonstrating how reasonable you are i.e. provided you are comfortable with it say you have no problem with them accessing your GPs notes
I'd say they were on a very sticky wicket trying to pursue a pregnancy related illness down the normal absence procedures, so keep that in mind and try to keep very very calm.
Good luck.