On wide open maps, they are basically forced to hug their corner and avoid overextending, because they get overwhelmed easily.
However, On city maps they are much more powerful thanks to Tommies having their bonuses on all the time and Vickers being terrors against buildings. They can easily leapfrog forward from garrison to garrison and unless you get lucky with mortars or manage to flush the Vickers out, it can be all uphill from there for the Brit.
Early trenches on cutoff/fuel/ strategic spot can be effective, generally a disruptive opening with IS in buildings is good as long as you secure fuel and get snipers, AEC, or mortar pit