(As far as the UK is concerned) the only person that can force anyone to delete pictures is a judge. If a policeman or anyone tells you to delete anything then ask to see his court order and refuse. Period. A security guard can ask you to stop taking pictures and leave private property but he needs a court order too to ask you to delete pictures. You can take pictures of private buildings from public property... a road, or path... look for the presence of double yellow lines, for example, to give you evidence that it is a public road, not a private one.
A policeman can ask you to stop taking pictures, but you don't have too. By all means tell him who you are and why you are taking pictures, but in theory (although increasingly less in practice) Britain is still a free country. Inevitably the new laws relating to terrorism will lead some of the Police to think they have a right to order you to stop. If they try to quote this law, then fine.. ask them on what grounds they believe that your taking pictures could be construed as being of use to terrorists. (Assuming you aren't one of course). The rest is new ground, and we'll see what happens....