I always have the same approach with this, and it allways works for me a treat. So far this has worked with the NT, many shopping centres, a palace in London, several galleries in london, the london eye (complete with a free ticket), several high profile locations in Paris, The metro centre, the BT tower etc. etc.. (im getting old I cant remember them all)
I call the day before (or on the day) and ask for the centre manager / security manager)
If I have time I write and then call
I explain who I am, when I want to come, what I want to do
I expalin I can produce my insurance certificates
I turn up in a decent suit, with a name badge and business card
I go to security, and explain I have arrived
I never get any problems. Even better, I often get access to areas the public are not allowed to (example - top of brunell building in swindon). Often security will make sure I /we are safe, and our gear is looked after
Only once I have been turned down, and that was because some other photographer was booked in with a high profile celeb, which is fair doos on my book
Moral of story - take a professional, communicative approach, and the door will open. Do exactly what you will said you would do, no more no less. Act professional. This has even bagged me 3 jobs, where the place has called me back in to shoot something for them