You can still get a Purdey, Boss, or Westley Richards to go along with your Holland's. Similar price range. A Westley Richards hand detachable boxlock starts at 74,500 pounds sterling. Sidelock is more.
Continental guns are less expensive, Merkel doubles start around $12,000.
Bailey Bradshaw right here in By God Texas will build you a double rifle for $16,500 and up.
https://www.thehighroad.org/index.p...k-dr-nearing-completion.856594/#post-11243847
There is a pretty good cottage industry in converting shotguns to rifles.
You need a GOOD shotgun, rifle pressures are much higher.
Best to start with a strong 20 gauge so the barrel spacing doesn't look funny.
Regulation: It is commonly said that double rifles are "regulated" to shoot both barrels to the same point of impact at some moderate iron sight hunting range, 80 meters or 100 yards.
But Ray Ordorica had one that shot individual barrel groups the same distance apart as the muzzles at all ranges, bullets flying parallel. Note: this does not mean the barrels were laid parallel, vibration and recoil still required the barrel bores converge a bit.