When making them we (UK) tend to use pectin as the thickening agent for jams/marmalades and gelatin for jelly. You can buy sugar with pectin added marketed as '

When making them we (UK) tend to use pectin as the thickening agent for jams/marmalades and gelatin for jelly. You can buy sugar with pectin added marketed as 'jam sugar'. Jam should be nowhere near as firm as jelly.