You can, of course, zoom and pan the map as you wish. Panning is most easily done by simply moving your cursor onto the map, holding down the left mouse button (on PCs), and "dragging" the map.
Clicking on the firm/base name on the right will also give you access to links which will display a zoomed-in aerial view for the base's location, either on bing.com (was live.com) or google maps. bing.com often provides a higher resolution view than google maps (though frequently older). Note that the bing.com view, which comes from Microsoft, may only work on Macs if you use the Firefox web browser.
Note that on the google zoomed-in view one *may* have access to a street-level view, if available at that location. Bing.com on the other hand, will usually give access to a "birds-eye" view (from a low-flying aircraft rather than a satellite).
NOTE: I recently noticed that the google zoomed-in maps had stopped working properly, apparently due to a change by google in which they carelessly broke existing maps. I have made some changes and the maps should work correctly now.
Asterisks on base names on the right indicate status - no asterisk means the location is accurate. Please report any inaccurate locations that you discover.
Also available: Map of Hire Bases in England and Wales
Many thanks to Graham Keens for finding the locations of the bases for the large multiple-base firms.