The National Collection of Aerial Photography, based in Edinburgh, holds over 26 million aerial photos of places across the world. They are all viewable online, and coverage of Britain is reasonably good
The Aerofilms collection of 96,000 high resolution images, now part of the Historic England Archive, sadly only available online in very low resolution. If you want higher resolution, these have to be purchased.
The Cambridge University Collection of Aerial Photography (CUCAP) is the result of airborne survey campaigns which were started in 1947 by the pioneering JK St Joseph. Since then the collection has grown to almost 500,000 images of obliques and verticals in black and white, colour and infra-red. Virtually the whole of Britain has been covered, with the obliques depicting a wide variety of landscapes and features and the verticals being of survey quality and so can be used in mapping projects. Sadly, only a m inority of images are available online, and the collection is currently unstaffed so little is likely to change. Neverthless, a valuable resource.
Not to be confused with standard Google Earth, or Google Maps, Google Earth Pro must be downloaded and installed on your device first. After which, you can use the historical imagery feature, which enables the user to go back through previous aerial photography sometimes as early as 1945