Seems fairly straightforward to me. Boiler installation must be done by CORGI registered folk, therefore not DIY. B&Q is a DIY store, ergo it should not sell boilers.
Who told you that? Anyone who is competent is entitled as long as its not for someone else whos paying you. If its your boiler (from B&Q for instance) and your house you can. Just get a Corgi chap to commission and test it, that way your guarantee will be valid and you know its safe.