Tara,
Returned after eighteen months with Essex County Council. Although not a SW, commissioner. What I did notice is: What looks like good pay, say 25,000 27,000 £ is nothing to live on, especially if you are single. Any area around or close to London, it's totally out of the question. Cannot speak to supervision, but with the posts so utterly underpaid, my best guess it, not so good. But, must remember that where is work, in whatever capacity, there are always spots and supervisors that are good, and some..not so good. Unions in UK are not a strong as here.
If you want to work, don't use an agency. They are making money on you. I did my entire relocation from HSMP permit, to accepting post with Essex on my own.
If you were to go, just understand how expensive it is there, and plan for it. US dollar is half of what the pound is, and don't see that changing anytime soon.
Holiday time is generous.
My experience in UK was totally worth every minute, good and the not so good. If the right post came around, I would go back in a minute. Miss my friends from there everyday. My grandparent's save one, were all from UK.
Helen