A few things for you;
At this point in time sheds & builders merchants online offerings are not quite as good as the rest of the retail world. This translates as if you want something cheaper you are best to do it in person, either face to face, email or by phone.
Any national builders merchant (TP, Jewson, Buildbase - who I work for) are all capable of selling you materials cheaper than Wickes / B&Q etc... However you may need to negotiate with them, make them aware of what they need to beat as the Wickes price, give them the total list of materials required.
Having an account in itself will not simply get you something cheaper. It is your potential for business / turnover & their ability to make a profit from you that will make them sell things cheaper.
Yes, TP do own Wickes, but it has nothing to do with the day to day businesses, they are very different.
If you wanted materials cheaper & are likely to be a repeat customer, develop a relationship with a builders merchant. Hope this helps.