I have found this info on the UK Food Standards Agency website:
When you are checking food labels, you can use the following as a guide to work out if a food is high or low in sugar, per 100g.
Look for the 'Carbohydrates (of which sugars)' figure in the nutrition information panel on the label.
High is more than 15g sugars per 100g
Low is 5g sugars or less per 100g
If the amount of sugars per 100g is in between these figures, then that is a medium level of sugars.
Remember that the amount you eat of a particular food affects how much sugars you will get from it.
These figures can't tell you how much of the sugars come from milk or fruit and how much comes from added sugars.
Sometimes the figure you see in the nutrition panel is a total figure for 'Carbohydrates', and not for 'Carbohydrates (of which sugars)'. This means the figure will also include starchy carbohydrates.
So, to get a feel for whether the product is high in added sugars you might also need to look at the ingredients list.
Added sugars must be included in the ingredients list, which always starts with the biggest ingredient first. Watch out for other words that are used to describe added sugars, such as sucrose, glucose, fructose, maltose, hydrolysed starch and invert sugar, corn syrup and honey. If you see one of these near the top of the list, you know that the product is likely to be high in added sugars.
Some foods that you might not expect to have sugar added to them can contain lots, for example some breakfast cereals and cereal bars. Other foods can be higher in added sugar than you might expect, such as tins of spaghetti or baked beans.