I agree with the poster above. Living in unite accomadation is likely to lead to you mostly living with first years, who will be loud and inconsiderate of the level of work you'll have to do. It'll also be a similar size to the house you don't want to move into. In the house, I'm guessing everyone will be a 2nd/3rd year, and they will all have their own work to get on with and be a bit more considerate.
If you could afford a studio flat with its own kitchen/living area, I would say get one of those. However, these tend to be around £100 more expensive a month than rooms in shared houses, and you often have to pay all your own bills on top, whereas in a shared house some bills may be included in the rent.
I think the best thing to do would be to discuss it with your current housemates. There are a lot of maybes in your post- Maybe one girl will want to live with her boyfriend, maybe the other girls will want to move in with some other people. I would try and get a clearer picture of what they actually want to do. At this stage, you could also raise some of your concerns about living in a big shared house, and say you are going to look at alternatives too.
If they do want to move in with the other girls, you could also organise a meeting with them, to ask about things like having your boyfriend over. I'm sure they will have issues they want to raise too.