You can buy potted coconut trees from B&Q but these are intended as indoor pot plants which live for a few years rather than there being any expectation that they will grow into full size trees.
If you want to try growing a coconut tree from a shop-bought coconut, you might be succesful but, like all seeds, your chances of success will depend upon the conditions in which the coconut has been stored and your ability to provide the right conditions to encourage growth.
More information here:
http://www.plantcultures.org.uk/plants/coconut_grow_it.h tml
Chris