One thing to consider is barrel length.
As British guns shrink in bore diameter, they often (not always) get shorter in barrel length. 12's are often 28-30", 16's 26-28" and so on.
This is not a hard and fast rule but long barreled small bores usually command higher prices, all other factors being equal. They are often considered easier to shoot targets with and lend themselves to longer crossing and driven shots.
Of course you may be looking for a short barreled gun in which case you are going to find the search a little easier.
One other option is to consider a sleeved gun. Done well this does not ruin the balance of the gun as some would claim and can give you access to an effectively new set of barrels, longer chambers and sensible chokes often for considerably less than gun with original tubes.
Take a look at
http://www.heritageguns.co.uk/Cashmore%2016%20BLE/Cashmore%2016%20BLE%20Details.htm.I am not suggesting that it fits your requirements and it is way over your budget due to its high level of specification, finish and restoration but it might give you food for thought. This gun was a 30"/2 1/2"/12b but built on a very light action. We sleeved it to 30"/2 3/4"/16b which gives it a relatively robust action, long barrels and excellent handling.
