jacket potato
A potato cooked in its skin (NSOED).

• Harry was just helping himself to a jacket potato when Professor Quirrell came sprinting into the Hall (PS6) (U.K. edition)

jam
U.S.: Jelly. (The word 'jelly' in the U.K. means what we would call 'jello' in the U.S.)

• "Want a jam tart, Hermione?"
  - Fred (GF21)

joint (British edition only)
A large cut of meat for roasting (AHD).

• A joint of roast pork was sizzling in the oven (CS1).

jumper
U.S.: sweater.

• a revolting old jumper of Dudley's (brown with orange bobbles) (PS2)

• "Percy sent back his Christmas jumper." (OP23)

• Next to Tonks was Lupin, his face pale, his hair greying, a long and threadbare overcoat covering a shabby jumper and trousers (OP38)