Sexton has too low range and splash damage. Even though it was described as unit with area denial capabilities it really doesn't fill this or any other role. German infantry can waltz right through the area which is under fire by sexton (or even hang around if there is any cover) where as any other artillery piece will make sure that there will be losses. You have option of course to use Concentration Barrage with valentine but let's be honest, nobody uses it because it's not practical enough due to Concentration Barrage having too low range, sextons having too low range and barrages can't be on cool down. Right now UKF struggles against german positions in 4v4 if there are no soviet or USF players with artillery.
Increase damage, splash damage and unit cost. Sexton should be a viable unit without gimmicks like Concentration Barrage.