In the 1950s and 60s at the end of empire many of these old double rifles were brought back to the uk and kynoch also folding they effectively became useless. No ammo, nor much use for them. At that time you could keep a shotgun on a shotgun licence, which you bought for a few pennies at the post office. Rifles required a firearms certificate, which was much more involved and you need to show good reason. We then had the deer act with min energy and velocity requirements.

So cheap double rifles were simply cut up. Many of the better ones - family heirlooms were bored out to a shotgun - 410, 28 or 20bore. 25 years ago there was a wonderful gunshot in Bath that had many such converted double rifles. Many were BPE of one sort or another. Most were being sold to American buyers for quail shooting. I remember handling quite a few and they still had the heft of a double rifle even though were a 410 or 28. Having just finished graduate studies at Oxford, I was broke so could afford any, even though non were more than a few hundred pounds. I did buy a ward patent single 410 though!

My advice would be to drop a liner barrel down one or both tubes - an Einstecklauf - in something like 7x57r.