Before one plunges headfirst into a discussion of whether a "fine gun" is worth more when chambered at 2 1/2" vs 2 3/4", it would contribute more to an intelligent discussion if the poster were to recognize that the question does not begin and end only with chamber length. Here's why:
Let's assume that the fine Purdey in question was made relatively recently, say during the 1990's. Made for a UK buyer, chambered at 2 3/4", and proofed at the then-CIP standard of 850 bars. Well, if someone on this side of the pond were to buy that gun used, those 2 3/4" chambers and modern proof would not mean that the new American buyer could then go out and start feeding that gun standard American factory ammo--which is, I think following HOS' logic, the reason such a gun should be more valuable than a 2 1/2" gun. Per the Birmingham Proofhouse, service pressure for an 850 bar proof gun works out to a bit less than 10,800 psi, as we measure pressure in this country. The SAAMI standard for 12ga service pressure on American factory ammo is 11,500 psi--which means that the new American owner of that relatively new and modern proof Purdey with 2 3/4" chambers wouldn't be gaining anything based only on chamber length, because American factory ammo would still be too hot for the gun in question.
If an American buyer had gone to Purdey in the 90's and asked them to build him a game gun in which he intended to shoot 2 3/4" American factory loads, they would have built that gun as a 1200 bar "magnum proof"--and, as such, it would have been heavier than the same gun built standard proof, even if both had 2 3/4" chambers. So chamber length in and of itself, even if we're talking about modern British best guns, isn't where the story starts and ends. Both Major Burrard and Gough Thomas stated that it was perfectly fine to shoot British "standard" 2 3/4" shells in guns with 2 1/2" chambers. The problem came when people wanted to shoot American "standard" 2 3/4" shells in 2 1/2" chambers--or, for that matter, in "standard proof" British guns with 2 3/4" chambers.
So we reach the conclusion that the only advantage 2 3/4" chambers have in a fine British gun is if the gun is also given what was previously a 1200 bar "magnum proof" (now called a superior proof)--if the buyer's desire is to shoot American 2 3/4" shells in the gun. Otherwise, longer chambers not only provide no advantage over a 2 1/2" gun, but may--if the owner doesn't know what he's doing--lead him to expose the gun to service pressures higher than those for which it was designed.
