My understanding in the matter is as follows:
Pistols on an S1 must be 24 inches total length, with a 12 inch barrel and a 12 inch counter balance on the back of the grip. These lengths are fixed. changing any of these measurements makes it a S5 weapon.
In general an S5 is only granted for humane dispatch of live stock or game (deer) - and you must be able to prove you really need it.
S1 pistols may be used at approved target shooting clubs, and all relevant UK firearms laws apply to them, etc.
The only exception are members of the Irish prison service, or MP's (barstewards) - who may face death threats, etc. They can have what they want.....
If you are found with a pistol not on license, then i believe that you will be sentenced to 5 years on the inside.
If you have a license for any sort of pistol, i would say, DO NOT CARRY IT IN PUBLIC.
Armed police WILL shoot you DEAD - no if's, buts or maybes - DEAD.
Why are you so curious about UK firearms laws?