This site has a conversion chart which might be helpful.
Unfortunately, it's not as simple as it ought to be.
US half-and-half is 12% fat. The closest to that is UK single cream, but that's 18% fat, so not an exact substitution.
Whipping cream in the US is 30% fat and 35% fat in the UK. Again, close enough, but not exact.
UK double cream is 47% fat and unfortunately there doesn't seem to be an equally fatty and yummy US equivalent. Those poor poor Americans!
