Once you've already paid, you have strong grounds to take them to court of they charge more on a pre-agreed deal.
On a side note, it wouldn't hurt to stick an "is" in the middle of that article title... I mean, seriously, "Retailer's PS4 Preorder Ransom Illegal Says Trading Standards" makes absolutely no sense at a first glance - and article titles should be all about giving the most accurate idea at a glance, or failing that at least try to catch my attention. Having such a nonsensical title is just off-putting.
On a side note, it wouldn't hurt to stick an "is" in the middle of that article title... I mean, seriously, "Retailer's PS4 Preorder Ransom Illegal Says Trading Standards" makes absolutely no sense at a first glance - and article titles should be all about giving the most accurate idea at a glance, or failing that at least try to catch my attention. Having such a nonsensical title is just off-putting.