You are right to stress delivery as an important part of the experience. Or, more to the point, as a way to encourage customers to buy from giffgaff.
Choice of delivery company is for some a key factor in who they'll buy from online and, for me, there are some online sellers that I would never use even though they may be cheaper than others for the item I'm buying. Indeed, just last week I abandoned a purchase when I discovered that a company's choice of courier was going to be Yodel!
I have no doubt that giffgaff's choice of courier works for them and doesn't mess up too often. I'd also guess that they are value for money but that's no reason not to also offer prospective customers a more expensive, premium option which some will take to ensure (hopefully) trouble free delivery.
Click and collect is another option but in that instance there's both the need to ensure that the courier can get items to the collection point reliably as well as finding a suitable partner with plenty of pick up locations. All that adds complications and expense.
On the subject of manually entering addresses; I seem to recall when this came up before it was there as an anti-fraud feature and, due to the potential loss with selling expensive handsets, something like this is required to ensure that the person paying is actually the person who's going to get the handset and that giffgaff are actually going to get the money!