The product will come under the Consumer Protection [Distance Selling] Regulation 2000. You can send it back.But the return cost would be your responsibility unless you can prove that it is faulty under the Supply of Goods and Services Act 1982 [amended].
I think LG have a policy that 1 pixel dead in centre of screen.2 pixels 20mm apart. 4 or more overall in screen area, will warrant a replacement. But be a little wary, as LG may send you a same model refurbished monitor as the replacement, as per their terms and conditions.
Different manufacturers have different dead pixel tolerances, some are more generous than others. But TFT's a improving all the time with better manufacturing processes.