As already mentioned it could be applied lawfully or unlawfully.
I understand that the legality of it was challenged and for the allocation of duties it was determined to be perfectly acceptable.
For allocating holidays it is not. hence the number of bizarre systems in place for sorting it (can't for the life of me work out how it's working in my office now!)
Imagine we were employees at Tesco and the senior man/woman got first dibs on which checkout they worked for the day, would you kick up a fuss about that?
Most people wouldn't because they can see that it's basically the same job.
OK, senior man might be a little more comfortable, further away from the main doors in the winter or closer to the air con in the summer but still sitting there for however many hours serving and engaging in chit chat with as many customers as he can manage in that time (If I remember rightly going back 10+ years there is a target speed for getting them through your checkout) and then going home at the end of his shift.
How is what we do not basically the same??
We have 8,7,6 hours to deliver as much as we can and then we go home. Some walks may be a little more comfortable but they should all be the same. If they are not it's down to the idiots who cut the job up.
Tesco man may be a little slower than his colleagues because he takes his time to do his job with care, doesn't break the eggs or smash the wine bottles. But home time is still home time, doesn't get anywhere by rushing.