All milk sold by retailers in the UK must be pasteurised, which as a minimum heats the milk to 72°C for at least 15 seconds. This is designed to destroy nasty bacteria, like Salmonella, and reduce the number of less harmful bugs, which would make the milk 'go off'.
Provided the milk is cooled quickly to less than 5°C & stored at that temperature , it 'should' last at least 10 days.
The pasteurisation process is highly automated, and is designed so that any milk which hasn't achieved the correct temperature, is re-processed, by being sent back via an automatic 'diverter valve'. The process also employs a manual enzyme check to verify the process, on a regular basis.
What I mean to say is, that it is extremely unlikely any UK produced milk is compromised in any way during initial processing .However, subsequent storage & handling are vital to maintain the shelf life - there are still bacteria present, which will multiply if they get warm enough, & anything over 5°C is sufficient for this to happen.
So, if a particular retail outlet consistently sells milk that spoils before the 'sell by' date, then it is likely they are failing to store it properly, and you should avoid buying any from there.
One moan here - why don't fridges have thermometers, or thermostats that have °C on them, not meaningless numbers, I bet many people have inadvertently been running their fridges nearer 10°C than 5°C, during the recent heatwave....
<edit> to acknowledge there are other legal methods for increasing shelf life of milk, but pasteurisation is still the most common.
