The reason is that it was realized that there is NO GROUND SECURITY IN THE WORLD THAT COULD HAVE PREVENTED THIS!
The weapons used are on sale beyond the security point at Heathrow! Atleast in the US we don't sell swiss army knives inside the security zone.
The reason is that though British security on the ground may be better, your inflight security was appalling. one nutter almost crashed a 747 by his lonesome.
I suspect that a requirement to operate into the US is now a locked cockpit and inflight security staff. Seeing all the "A locked door wouldn't help" threads here, I am sure that BA and VS initially balked and said its not necesary. To which The FAA would have responded, okay, its not necesary for you to fly.
Cheers
Wino