UA and others usually require you to show up at checkin to collect all the data and reissue the boarding pass. I once managed to transit using an AI boarding pass and luckily showed it at the LH lounge. It took the lounge attendant about 15 min to straighten out things. COnsidering UK probably uses the state of the art Altea suite for checkin and UA their ancient Shares there is a ton of stuff to do...