As for the phones, I don't know a single person who buys their phones outright, let alone from ebay, so to quote £239 for the purchase is misleading.
Similarly, the Nuvi comparison is a red herring. An in-car GPS is competing with phone GPS units much more directly than a Garmin Edge of any iteration. Power is not an issue for the phone, function is everything. And it is a pure navigation tool.
The Nexus 4 is £239 direct from Google, not misleading.
My bad - too early in the morning!
I don't know why the Nuvi comparison is a red hearing? They can operate on battery, and they have to a drive a 5" screen. The internals past the screen are of a similar constitution, and they use the same mapping service.
Battery life on the Nuvi is irrelevant as most, if not all the time, it'll be plugged in. It only has to do one thing, navigate, not record speed, distance, HR etc. it has no other function other than to direct you to a destination. It doesn't have to be weatherproof, nor shock resistant, so of course it's cheaper.
Come to think of it, imagine if that £239 Nexus was waterproof & shock resistant. How much bulkier, heavier & more expensive.