The best British guns were said to have been built between the wars , @Ravers may be able to confirm this .I'm afraid that you slightly lost me at '100 plus grand'. To those who can afford that kind of money for a gun (or is it a pair?), the cost of replacement barrels would be an inconvenience rather than a calamity, or so it seems to me.
The people that own them are not multi millionaires , they're people who on the whole have inherited them , and may be asset rich , hardly ever cash rich .
The value as much as anything is in the originality of the gun . Replacement barrels do not add value , but rather detract and these people will not for k out £,000's for a replacement
I suspect there will be an interesting illicit trade in lead cartridges for a few years