I will give the explanation I have been given and read a number of times over the years about the lay out of a Brit traditional side by side. First of all, it is nothing to do with the barrels or choking it is all to do with practicality and tradition thrown in and I am sure you are aware that we Brits run on tradition. If you take a single barrel gun in the far past gunmakers always without exception put the lock on the tight hand side because everyone in their eyes was right handed being a lefty had lots of bad associations Devil etc, even to Edwardian times children who were obviously left handed where made to change to being right handed with disastrous results in many cases. That being the case the cock hammer or the part that held the glowing match was to be found on the right side of the guns lay out. With a single barrel gun, no problem lock on the right and one trigger and you are right handed well the gunmaker says you are. Then two barrels came along two locks one either side of the gun and you only have one thumb to pull the cock or hammer to half or full cock, and not every person who shoots has a hand the size of a coal shovel so to raise the right hand cock or hammer is easier because your thumb is in a straighter line with your wrist, the left hammer is a more oblique pull and to many people more effort is needed (this does include myself) this being the case the right hand hammer or cock was favoured and in consequence the right hand barrel because of the guns physical layout. It is also said that on walking up game the bird when flushed is closer to the gun so the first barrel you use requires less choke but that came a lot later after the design was set.
Also, on many vintage Brit side by side guns you can change the barrel firing order just by swapping the triggers over though not all. On occasions this can be a benefit, the odd time I visit a friend who lives a good distance away who is a left hander I can borrow one of his guns because it has neutral cast and just do a quick trigger swap.
But like all things in life there may be a lot more explanations though I do like this one.

The only lessons in my life I truly did learn from where the ones I paid for!