Grens late game if they dont upgrade to LMG are worthless
Volks late game if they dont upgrade to StG are worthless
Obers late game if they dont upgrade to LMG are worthless
Riflemen late game if they dont upgrade to BARS/LMG are worthless
Guards late game if they dont upgrade to LMG are worthless
Panzerfusilier late game if they dont upgrade to G43 are worthless
So yes, Unupgraded british infantry if they dont upgrade to LMGs in late game they are worthless
Let's break down the IS and see what you're comparing them to.
The Infantry Section is a T0 British main line infantry that needs to hug cover or else they're dead, compared to any infantry unit (except for the Osttruppen maybe) they can't even survive crossing a road to get to cover and will lose more models than any other squad for doing so. If caught off guard and out of cover, they're ineffective and effectively dead, like I already mentioned.
They cost 280 man power to produce and 25 to reinforce, starting out as a 4 man squad that can be upgraded to a 5 man one, requiring even more resources to improve their effectiveness, then you have to additionally pay for any squad already on the field to get that extra squad member which brings the price of any squad BEFORE the upgrade at around 300 man power. Which leads to the main weakness of the British and that being their man power bleed and less squads on the field at any given time, this is also one of the reasons we almost never see more than 2 IS squads and people spamming Sappers and emplacements, Sappers are a hell of a lot cheaper while emplacements may not be cheap but at least they don't bleed you like IS do.
Further more they require the unlocking of Grenades (The mills isn't really effective compared to the other grenades in game) and the weapon racks, each Bren costing 60 ammo while being less effective than the Gren's MG42 LMG upgrade for example, plus their optional side upgrades being the pyrotechnics/medic supplies that's around 150/160 ammo invested in a squad IF you have double brens and either upgrade on them, and I'm using double brens as an example because let's face it, the PIAT is the worse hand-held AT weapon in the game and people generally don't really use it on any other squad besides the Sappers from time to time.
Further more their Vet 3 is actually working against them rather than for them, meaning they get less out of their top most veterancy rank compared to any other infantry unit, especially the Riflemen's survivability buff at Vet 3.
So, compared to Grens, they're kinda balanced when both are in cover and long range early game, the IS lose to MG42 Grens even when they get a 5th squad member, and the Grens lose to IS if they have the 5th man upgrade as well as a Bren on their side, they completely get destroyed by a double Bren 5 man IS squad with or without cover, not to mention of the IS also have the medic upgrade. That's one of the reasons why Wehrmacht find it so effective fighting the British with support weapons (Snipers being most effective because of the bleed they inflict) rather than their mainline infantry. Now don't quote me on this but I saw a video before of the Grens getting pitted against the IS in one such scenario like my example, that's where I got it from. And yes, the Grenadiers do need a buff if you ask me, a 5th man would do them justice compared to now.
Rifleman, against a 280 squad but arguably better in almost any way, all 5 members are armed with the M1 Garand which is superior at Close to mid range to any other mainline infantry squad, get smoke with the unlocking of their grenades, double BARs and veterancy 3 can even enable them to fight Obersoldaten, I also forgot to mention they get AT snares with veterancy 1. The only thing the IS have over these guys is their medic upgrade.
Volksgrenadiers - Still cheaper than and more spammable than the IS, get a cheap close to medium range effective upgrade that helps them scale as storm infantry later on compared to the IS, 5 man squad right from the get go, no upgrades required for them to achieve their full effectiveness at long range. They were even better before with the Panzershreck that helped them vet really fast compared to now. Oh and I also forgot, 5 vets.
Cons - they're shit, I don't even play Soviets so I really can't comment on Soviet infantry but I think being a 6 man squad they get wiped a lot less than the IS and don't bleed man power as much, Soviets also have a lot of doctrines that makes Cons pretty good support units and gives them more utility so there.