The problem with buying goods from Pixmania is that your order is placed through their French Website, Pixmania being a French based company. The rule of thumb with buying online is that the law of the country where the selling company is based is the one that applies not the law of the country where the purchaser resides.
As France is part of the EU hopefully they will have in place a French version of the Distance Selling Regulations (DSR) as it is I believe a piece of EU Legislation. Therefore you will have your 7 day cooling off period. This period starts on the day you receive the goods providing that by the time you receive them the seller has told you of the existence of these rights. This is normally done by including something in the paperwork that comes with the goods or by email from the seller confirming your order. If the seller does not inform you of the rights the cooling off period is effectively extended by several months.
Firstly I would advise that you contact UK European Consumer Centre for advise. This is run by trading standards and can give you advise on European Consumer Law Issues.
ECC: Trading Standards Institute
If the goods are faulty then you will have to pursue any claim under French Law. However if the goods cost more than £100 and you paid for them using a credit card you can under Section 75 of the Consumer Credit Act make a claim against your credit card company. So it might be worth contacting them.
Again if the goods are faulty you could consider a claim under the warranty/guarantee if the manufacture gave one. I would tend to only do this as a last resort as your rights would be limited to the terms and conditions of the warranty/guarantee
Finally people have made comment about the wording of Pixmanias terms and conditions (T&CS). When dealing with a UK business it is possible to challenge T&Cs if they can be shown to be unfair. This may or may not be possible in France.