You can just get an extra unit instead of bolster.
That doesn't work due to how the popcap system works.
Popcap for infantry is squad pop value + 1 per model. Regardless of cost of the upgrade, it will always be more efficient to get 4 squads of IS with Bolster than 5 squads of IS without it.
And this doesn't even account for RE.
If we account for either pyro/medic (now even the raid upgrade) on top of the weapon system things gets even uglier.
The problem has always been 5 man IS with either squad upgrade, grenade unlocked and double weapons been over the top. But to achieve that, you need to spend an unholy amount of resources and you will only see it when at really late stage of the games or as a consequence of a snowball effect.
In regards to the weapon system, i think the issue is that the Bren was nerfed in the wrong direction. A single Bren doesn't cut it and if we account for the penalties out of cover affecting way more the performance of LMGs than their base rifle, you would rather have an extra model of IS rather than a 4man squad with a single Bren.
Finally, vet. It was the initial general design that UKF would get worst vet bonuses on their infantry, so their scaling is more tied to those upgrades. If you check, you would see that the bonuses are inferior to all other factions main line bonuses. Reminder that they had their scoped lee enfields at vet 3 removed, cause the execution of it was extremely poor (they increased the likehood of dropping weapons and, at first, they even reduced the DPS of the unit compared to vanilla rifles). Even after it was fixed it was barely an improvement.
TL;DR: Bolster is problematic because it's the more visible and worst component of a whole system which is flawed.
If you asked me what makes IS hard to balance it is in order:
Bolster >> Weapon upgrade >>>> Squad Upgrade >> Sandbag > Vet > Trench > Grenade >>> Millbomb