I'd have to say that my experience with Heathrow was altogether very different
When flying in from Aus via Dubai I showed my passport in the non-british section and said I was on holiday
Following that I walked to the baggage carousel, grabbed my stuff, walked out to the bus stop and caught a bus to London
Now given that this was post-911 and post-omgwaterbottles I was incredibly surprised that at absolutely no point was I subject to a security check
I was travelling on an Emirates flight from Dubai and on this flight were a fair number of what the US govt generally refers to as 'high possibility terrorists'
So why no-one on our flight was subject to any security check is well beyond me, I'm guessing the football was on tv or something ....
In contrast when leaving Heathrow and travelling back to Aus I was subject to the third degree for having some dutch cheese in my carry-on luggage ... which I told them about ...
I was made to wait for half an hour while they harassed some german woman over having keys in her back, which she had tried to show them however they dont let you touch your own bag around the security section
Imo if someone really wanted to make a public display of killing people it wouldnt be on the plane itself, it would be at the outgoing security checks at Heathrow
You would certainly create more fear and kill more people due to the inconsistency and snobbery of the british security system
In summary
Entering the UK and killing many people = easy
Leaving the UK and going somewhere where you might be less inclined to kill people ... = hard
UK = nanny state with schizophrenia