I just checked a random monitor from Overclockers, absolutely nowhere in the description or features does it mention anything about a dead pixel policy. I disagree that you are accepting the monitor will meet a certain dead pixel standard when there is zero mention of it at the point of sale.
In any case the actual law disagrees with you here no matter what may be written in the product description. You are allowed to open and test the goods as long as you don't mishandle and damage them, and you are entitled to a full refund including standard delivery costs, the exception is if you paid for express delivery which was not the cheapest option.
34.
(2) The trader must reimburse any payment for delivery received from the consumer, unless the consumer expressly chose a kind of delivery costing more than the least expensive common and generally acceptable kind of delivery offered by the trader.
(3) In that case, the trader must reimburse any payment for delivery received from the consumer up to the amount the consumer would have paid if the consumer had chosen the least expensive common and generally acceptable kind of delivery offered by the trader.
(9) If (in the case of a sales contract) the value of the goods is diminished by any amount as a result of handling of the goods by the consumer beyond what is necessary to establish the nature, characteristics and functioning of the goods, the trader may recover that amount from the consumer, up to the contract price.