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 27β which is 68.58mm. 70mm would be 27.56β so 27β is a slight compromise. 29β is 73.66. Some English makers used 29β to differentiate their guns being slightly lighter. 25β was the brainchild of WWGreener which is 63.5mm. The metric to inches is a bit messy. You see a lot of 27 & 29β 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&29βas well.
There was a time not too long ago which the prevailing wisdom was 30β is too long for a fast handling double and 25-26β was where the market was. Now guns with 30β seem more sellable and those with 32β are even in more demand. 25-26β are very hard to sell. I like double more for handling, feel if you want, than any flat statistics.