difficult...missing family in the us a bit and starting a family in the uk

If you feel you'll need a support system that strongly, then while you're pregnant you could start creating one, via online groups, antenatal classes, etc. Like Lilybelle said, your support group doesn't necessarily need to be your parents or families. If you have a special needs child, surely you could reach out to groups and parents also dealing with these same issues.
Of course it's wonderful if your family can be involved, but as some people have posted here unfortunately some people can't or aren't interested in being involved.

And bookgrl, all of those things you listed can happen in families that DO have support as well. There are no guarantees.

Oh I know, but it is just more likely to happen to those that don't and I have neither family or many friends here.  I know support groups can be made, but it is nice to have a base to start on. 

Just to clarify - even from long distance my family has been a support to me here.  I also have found some good close friends who will babysit and have a great relationship with my son. So please be assured that you can find a support system locally even if your family is not supportive. 

The most important source of support though is my husband, who participates fully in the process of raising our son.  I think many styles of families can work and create a loving environment for raising children.  As long as you have love and support in your relationship with your significant other, you'll be able to make a way.  Children definitely put additional strain on that relationship, so that's probably what I would be most concerned with before starting a family.