Think the law in UK says that you are entitled to return goods without reason, for up to 14 days.
The other thing is that your eyesight would be keen indeed to see one dead pixel. It is more likely ten or so, in which case the manufacturer will exchange.
Don't take any cr@p from retailers. Do a bit of checking with the Trading Standards Office first and then raising the battle flag, march in the retailers' direction.
good luck!
Conan