Not necessary... FS2002 has more UK airfields on than you will ever need. If you want more detailed scenery on some particular airports however, try finding some freeware files on the web.
Although I've not seen it in action 'Get-Mapping's UK aerial photo scenery for FS2002 is supposed to be worth getting - see their website for detail (although I dont think this adds anything to specific airfield scenery).
Just a general point, make sure your hardware is well over the minimum required by Bill on the box, if not, you will find yourself having to turn off much of the detail to achieve an acceptable frame rate.
Tim