The other factors are age and eyesight. Most shooters in the UKicon are 50+ their eyesight is going and so sticking a PH 5a on an SMLE for instance is really the only way they can keep shooting a SMLE. In some cases this applies to their No4s.
Also a number of shoots, such as the Scottish Classic Rifle Championships, are running seperate comps for target rifles of that period so it's quite easy to get into 2 comps insted of one by removing the sight.