Originally Posted by
Airprox
At JER they will only be checking in your luggage to LGW where you will have to claim it all and take it to LHR with you, so when you check in at LHR you are essentially starting again and can check in as much or as little as you like subject to your allowance.
But even if it was a same day and airport connection (eg XXX-LHR-YYY) you can check in additional luggage at the connection point assuming you meet the checkin cutoff time.