What is wrong with replacement barrels? I would prefer them over sleeving. The British take a whole different look at things than we do here in the Sataes. To them the gun is to be used. Most of these early guns were uaed a lot and often on many shoots during the season. Most fine London guns were originally made with thin barrels for balance and getting the weight down under seven pounds. So it is only natural that a gun that saw action several times a season and shot hundreds of rounds each shoot, then the barrels would wear out.
Now I agree it is nice to find a Boss or Purdey made in the early years with original barrels and plenty of wall thickness. But lets face it, most barrels by now are in the low 20's on wall thickness.
Replacement barrels done right are much better if you are wanting a shooter than weak, thin original barrels. How much is a set of new barrels going to cost by Purdey, Holland or Boss?
Sleeved barrels will certainly bring shutters to die hard purists who only want the very best.
It's time to realize most of these guns are getting to be or are already over a hundred years old.