By "top tier English gun" I suppose you mean a best quality SLE from Boss, H&H, Purdey, or Woodward. Such guns are valued in the USA retail market at around $15,000 in heavily used, but not abused, current condition. That is a right fair chunk of change for a heavily used gun. Since the market supports this value, we have to believe such guns are both scarce and highly desirable. So, who buys such guns?
I divide the market's buyers into Collectors, Collector-Shooters, Shooter-Collectors, and Shooters. "Collector" is motivated by attributes other than his own shooting. The "Shooter" is only interested in shooting quality. The $15,000 price moves these guns into the Collector-Shooter range, as it is hard to ascribe $15,000 of shooter value to a heavily used gun. The Collector-Shooter finds a balance between the desirability of the gun in high condition vs its remaining shooting quality.
Replacement barrels, even by the maker, takes a whack at the Collector value as Original/Pristine current condition is so highly desirable. The Shooter, on the other hand, uses current condition to asses remaining shooting quality and economy. The barrel condition is one key to the overall condition of the gun. New condition barrels makes this assesment more difficult. Replacement parts always beg the question of, "Why?" "Has this gun been so heavily used as to wear out the original barrels?" Or, "Did the original barrels meet some untimely fate?" "Are the new ones as good as the OE?" Any time there is a question, good answer or not, the price goes down.
Using the above as a framework, I plan to further study this question based on London auction results. Most auctions have several guns that fit the assumed discription, so "comparables" should be easy.
Good question!!
