CCTV cameras used for domestic security aren't covered by data protection laws. The same laws (or lack of them) apply as to any other kind of photography when you're own on your own premises (or in a public place, or in a place where the owner has authorised photography). i.e. as long as you steer clear of offences relating to such things as official secrets, indecent photographs of children and 'voyeurism', you're free to take pictures (including video images) of whatever or whoever you want to, and to do whatever you like with those images.
So you're free to install as many cameras as you like in or about your house. Where they point at is completely irrelevant. You can lawfully video passers by in the street, your neighbours in their garden (or, if they leave their curtains open, in their own homes), or anything else you like.
http://www.ico.gov.uk...FCD-98A2-278DE2D7446C
Chris