As in my first post, if a BV1 (Boss, H&H, Purdey, & Woodward) branded gun is valued $10,000, BV2 (17 Brit makers so valued) would be $7,500, and BV3 would be $5,000 for the same quality gun. If the gun is, in fact, the linked gun which is 16 gauge, then you need to add a premium of maybe 1.25x to 1.5x (note that I haven't done a 16 bore vs 12 bore recently).
I would suggest that if this gun were a 12 bore, valuation would be CC6 (Shootable but needs some repairs and refinishing or refinished with barrels or stock replaced). Top end can't be more than CC5 (Minor repairs needed or proper restoration accomplished). BV3-OQ1-CC6 = $4,500 and BV3-OQ1-CC5 = $6,000. The about $25,000 price tag suggests BV3-OQ1-CC1 (Pristine = top one or two gun(s) of one hundred like guns) = $20,000 plus a 16 bore premium of $5,000 (1.25 X).
Note that value is a hypothetical number whereas price is an amount accepted by both a willing and able buyer and a willing seller.