William Powell is a Birmingham gunmaker that is still in business. Go here for more info including valuations:
http://www.william-powell.co.uk/comp.htm
Shotgunning guru Michael MacIntosh in his book Best Guns (Revised Edition) had this to say about William Powell, "Powell guns of every period, from the 1830s to the 1990s, show all the craftsmanship and handling dynamics that best guns should."