Don't remember seeing this here before so thought I'd add it. Britain from above is a collection of period aerial photography from a 'birds eye' perspective. It's not a complete survey and therefore a bit hit and miss although large cities are covered well. You can register and this allows you to zoom in to the photos without losing much resolution.
It's an excellent resource in my humble
http://www.britainfromabove.org.uk
It's an excellent resource in my humble
http://www.britainfromabove.org.uk