Forat, half&half is a milk product with 10% milk-fat (in Canada anyway. Apparently it typically has 12%fat in the US). As AW indicated its effectively 2%/part-skim-milk mixed with equal parts of 18%/table/coffee cream. If you're in the UK I think it might be closest to your 'Single Cream' but a bit lower in fat.
BTW - I'm going to try this recipe today or tomorrow - supposed to be a HOT wknd & this sounds A-MAZ-ING!