I've just biked from my workshop in Hoxton to the Gherkin, 30 St Mary Axe. A good day to do it, though the weather is cold and blustery. (Call this June? Huh!) Few cars and vans, just red buses careering around in packs with a gleeful freedom non-Bank Holiday traffic does not allow.
I went to get a feel for the building close to, and take some photos, as part of my WIP is set there. In my fictional 2018 London, twenty metres of snow cover the city, and only the taller buildings are visible. One of my characters conceals his snowmobile inside the Gherkin. I've studied the website carefully. Of course, what I'd really like is to wander round the building, camera in hand, but this is not possible. Luckily, it's not necessary either. I can make it up.
Because in fiction, you don't need to be right, just convincing. I don't know any Security Service officers, but no readers have yet complained that Nick Cavanagh in Replica is an unbelievable MI5 spec op. Probably because they don't know any either. Nor has anyone said the scene set in the Dorchester in Remix with information gleaned from their website and Google Street View lacks verisimilitude.
My advice: do all the research you can, go to the places that figure in your books, talk to people doing the jobs your characters do if at all possible - then make the rest up. Do it well, and nobody will know.
N.B. If any Security Service Officers are reading this and wish to put me right, email me - I'm very discreet :o)