As an American that moved into her British boyfriend's parents house (for 1 month). . . my advice is do it only as long as you need to. No matter how great your parents are, it is really difficult to live in someone else's house. I was so thrilled to just be with him after months of long distance, but it was not ideal living with his parents. Remeber, these are people you've lived with for your whole life, so it won't be automatic for your fiancee to be on the same comfort level as you. Your fiancee is going to be going through a lot when she arrives!
I'm not sure if your fiancee will be able to work. I think it will be much easier if she can. . . When I moved here, I wasn't able to work (I still am not) and I was waiting to hear back from the Highly Skilled Migrant Programme. At first, we were going to wait to find out the results before we moved out. . .Thank God we didn't! It took 8 weeks to get the results, and I was rejected, so if we had waited we might not have gotten a place. To be completely honest, I don't think I would have made it for 6 months in his parents house. Also, most landlords offer 6 month leases and when we got our place, I only had 5 months left (so if we waited any longer it would have been silly).
For me, it was just a matter of not feeling like a slob if I want to stay in my pajamas until noon (ok, or for all day). Or just being able to read a book in peace, without parents worrying that something is wrong with me. I didn't feel comfortable just whipping up a meal for myself in the kitchen (my boyfriend didn't understand this) but food is very different here than in America. I think that most families here have staple foods that they prepare (spag. bol., fish pie, shephards pie, Sunday Lunch etc.) and in America every house has something different. It was hard to explain that just because I didn't know how to cook the English basics, didn't mean I don't know how to cook. (His mom did all of the grocery shopping, so it wasn't really my territory). Laundry is another issue. I know that people have written about it in the forum, but you'll have to be aware that its not that she doesn't know how to do it, but laundry is very different over here. Until I came here, I had never seen anyone iron wet clothes! I didn't know what temperature (celsius or farenheit!) to put clothes on, and I had no idea where to put the powder!
I also found myslef avoiding arguments at any cost when we were in the house. It is really hard to have a fight or argument (even a discussion) when there are constantly other people around. My advice is to make sure you give her extra attention, and make an effort to bring her out for alone time where she can tell you what's going on in her head while you're working!
We now have our own flat and I am so much more comfortable. Worth every penny! Good luck!