My experience of LHR: BA check 95% of the time on boarding a/c, bmi hardly ever, like most other carriers they check only at the top of the airbridge.
Seems like overkill to me as the chances to slip up the stairs on the airbridge unnoticed are limited, and without a boarding card, there would be a little trouble locating a seat on board.
If you manage to get airside, surely it's better to slip something in the cargo hold or on the undercarriage, rather than draw attention to yourself by boarding the a/c without a boarding card? Then again, if you did manage to have a forged boarding card with a correct seat number...... you'd come on via the main door!!
(typo)