Truth is British gunmakers would give the customer whatever he was willing to pay for. It is a metric issue, or conversion by and large. A lot of Spanish guns are 27” which is 68.58mm. 70mm would be 27.56” so 27” is a slight compromise. 29” is 73.66. Some English makers used 29” to differentiate their guns being slightly lighter. 25” was the brainchild of WWGreener which is 63.5mm. The metric to inches is a bit messy. You see a lot of 27 & 29” Spanish guns which were common factory legends unlike American guns which would almost always have been cut down to be those legends. German and Italian guns are commonly seen in 27&29”as well.

There was a time not too long ago which the prevailing wisdom was 30” is too long for a fast handling double and 25-26” was where the market was. Now guns with 30” seem more sellable and those with 32” are even in more demand. 25-26” are very hard to sell. I like double more for handling, feel if you want, than any flat statistics.