Yes Broadgate was the place! Is it a security thing or what?
Essentially, the 'estate' is private property and that will often include the 'open space' around the offices and shops (if there are any).
The security staff can ask you to stop taking photographs from positions on the private land owned ny the company owning or leasing the estate. They cannot, however, stop you from taking pictures from the public footpath/public highway. Look for markers on a boundary between the public footbath and the site. They can be as simple as a row of different coloured paviors or a row of metal stus set to outline the private/public boundary.
A lot of 'security' staff have orders to reduce pictures being taken of staff working in buildings - which one of the offices I was doing an environmentsl review on, which included my needing to do detailed photography on the building itself, meant I was asked to do the photography on a Sunday.
A good bit of London's main attractions can prove to be difficult, especially if you are using a tripod! I even had an issue using a very thin monopod on a site in Bristol.
Shopping centre owners/managing agents have the same approach with photography.
Worth remembering that public organisations, government and local authorities, are often located in privately owned offices/estates so you cannot assume you have a 'right' to use your camera by stint of being a tax or council tax payer.
Still there are loads of ways to 'skin a cat'. If there is a particular place you want to shoot at, you could always contact the owners/managing agents.
