You're right, the vaccines might help with disease severity but they don't protect the person from infection if they are exposed to a sufficient amount of virus.

As for the transmission of the virus, according to preliminary info (so take with a pinch of salt) the vaccines *might* help to reduce the transmission rate of the virus. They think people who have had the vax have reduced viral loads therefore making them less contagious to others by shedding fewer virus particles.

So I guess Gov's argument would be that yes it's probably inevitible you're going to be having people spread the virus in venues but taking the vax *might* make you less likely to spread the virus to others thereby helping keep the R rate lower.

The way I see it, UK Gov will wait a while to see if the R rate reduces on its own, if it doesn't then they will make vaccine passports mandatory for venues, shops, businesses etc. It would take the form as a type of ID card you carry everywhere with you and scan at venues. Like I said the other day people like Blair etc would love this because they were pushing for ID cards years ago, but they never got it because of pushback from civil liberties groups. But of course you wouldn't need these vaccine passports at all if a zero-covid strategy was implemented, but the UK Gov has already resigned themselves to not pursue that policy.