No one will be able to answer this, but the two of you!
Family support is huge, in my opinion. I've had a child since moving here and while my inlaws are lovely people, they are not involved grandparents. Fortunately we have now found a couple of babysitters and are able to occasionally go to dinner just the two of us.
As SoS says, it really depends on where you are moving from and where you are moving to. I moved from Denver Colorado, was single, and making an excellent salary for the US. Moved to the UK for work and while I thought I had a great salary in the US, I was eating into savings greatly my first year. I moved to Wokingham, which is in the SouthEast.
Where we live, to get into a detached house (single family home) it will cost a minimum of £550k ($800k). We are fortunate to be able to afford this and now have a detached home! It is 1200 square feet. Up North, this would buy a mansion from what I understand.
For most people, salaries will be substantially less in the UK and the cost will be substantially higher. But there is more to life than "numbers". My career has been very successful here (international finance). My husband and I do very well salary wise and therefore we can afford a trip home to the US each year, as well as a summer holiday outside of the UK. We eat out about once a month (two fold, it costs us about £70 to eat out. And my husband grew up NEVER eating out, so it's a compromise with us).
I don't know if you have children but I'm nervous about how my daughter will do in the UK school system. It is strictly exam based. I worry that if she is not a good test taker that she may not succeed, whereas the US system is more "rounded" with tests, homework, assignments, reading, essays, projects, presentations, etc.
I'm happy with my life here. I was happy with my life in the US. I think people think that my life is glamorous. It's not. It's the same life (work, bills, positives, negatives) that I lived in the US, just in a different location.