In all probability a thorough pre flight inspection was carried out at LHR prior to departure by engineering/flight crew and indeed at the down route stations that the aircraft previously visited. The fact that a stowaway remained undetected does not mean these checks were actioned without due care. On most large passenger aircraft, with the exception of the B737, it is simply not possible to see into the wheel wells from ground level with the gear doors closed. Even with access steps positioned your view is still limited in many cases. The only sure way to see is to open the gear doors and this is not usually a routine requirement on pre flight/transit checks.