16 British doubles are a lot rarer than 12's in my experiences. I would expect to have a least 25% premium and would not be suprised at a 50% asking price extra. Asking and getting are not the same.
One other important thing other than the short chamber of British doubles when compared to US doubles is the weight issue. A 12 British double will weigh less than a US 12 and about what a US 16 weighs. A 16 British double is in the US 20 range. So if you are looking for a 7 pound double think British 12 or US 16. If you are looking for a six pound gun look for a British 16 or a US 20.
