B & Q, Argos, Homebase. I think these are all the same shelves, Argos and Homebase are the same company and seem to carry much the same stock.
Anyway, I have attempted to make large floating shelves 1.4m long and 32cm deep out of 26mm oak veneered MDF. [broken link removed] Using these [broken link removed] 3 per shelf, as support. Plus 7 or 8 inch masonry screws.
It is not work as the wall was dot and dabb plasterboard on block, and the shevles too heavy and thin. There was too much give in the plasterboard, which allowed the shelves to bounce too much. Even though the supports/ brackets could easly take the weight of a man. I ended up putting in additional supports, rather than spend time messing with injecting filler behind the plasterboard.
If you look at the shelves in the shops, they are small and filled with polysterne (sandwidge) to keep them very light. They are also very thick to give them support against the back of the wall, ie less bounce. If I was doing it again, I would build my shelves the in much same way. Using two thin sheets sandwidge between a thick sheet of polysterne, with internal wood to support the brackets etc.