The offence of possession of an offensive weapon is only complete when not at your place of abode. Therefore this doesn't apply. Having a samurai sword for example above the mantelpiece is okay (from a legal point of view, taste is another matter). Furthermore, an offensive weapon needs to be made as an offensive weapon (e.g. knuckle dusters), adapted to be one (e.g. a shortened pool cue hidden up your sleeve) or intended to be one (where this torch business comes from I guess). Carrying about a Maglite after dark isn't carrying about an offensive weapon, unless you have the intention to use it to brain someone. It's a torch and is used to produce light. Yes, the bigger ones are heavy and well made, but the intention there is only to make them hard wearing and reliable.
As for the choice of the Maglite, the "C" and "D" refer to the size of the batteries used. The old ones are okay, but the newer LED ones are much better in the amount of light they produce and the batteries last a lot longer. I think they remain popular because they are so well made, but you may well find equally good or even better LED torches for the sake of a bit of surfing on the internet. Look at those able to be stuck on your rifle scope for example.