How exactly is lacking a mortar any different than lacking an MG? This is a key teamweapon every faction has.
You could argue the "defensive Brits" design went out the window when trenches were nerfed, Tommies got moving accuracy buff and their in cover RA got stripped. (Or do you prefer static play and emplacements?)
I doubt the doctrine will be used personally, tactical support gives better options and a few muni dump abilities that aren't smoke.
brits dont lack a mortar, they lack a mobile mortar. there is a difference. one could argue that the kuble filled that role, and it sort of did.. but it was far too strong in the early moments and fell off very rapidly until being completely unusable once any AT hit the field.
the brit mortar is static, but if placed conservatively can provide fire support on the enemy front via barrages and autofire when on the defensive.
the brits are still more defensive than any other faction. less so that previously, but still more static and defensive by a large margin. nerfing the trenches if anything made them more defensive and less offensive as you couldnt throw up a strong point in enemy territory. the issue is people still want the brits to play like other factions where they are disadvantaged instead of playing to their strengths which is drawing a line in the sand and probing outside of it instead of trying to keep the enemy off balance with constant unpredictable pressure like the usf.
the only overlap with tac support is the mortar. you still have a slow lumbering expensive FF at your only TD option and sappers as your cqb infantry, you aggression which this new commander grants. tac support grants new ways to hold the line, not new ways to break one. there is a niche for this commander that isnt available elsewhere, closest would probably the the land mattress commander but despite the overlap they both offer vastly different approaches to urban combat and taking ground.