Maybe once they get their groceries business going it might be worth it, but he current [GBP]80/year is way more than it would cost to just pay for next day shipping on the stuff I care about
They're going to have a hell of an uphill battle with breaking into the groceries delivery market. The competition there is *feirce* between Tesco, Sainsbury's and Ocado, not to mention the slightly more minor players (going by how much I, personally, see vans out and about---it's probably different in other parts of the UK) like Morrisons, Asda, Waitrose and Aldi. Apparently Lidl is already following suit.
And of the ones I've used, the service is really good[*]. I have never had a late delivery. Those companies already have massive countrywide logistics networks, geared up for the delivery of chilled goods as well.
They already use the internet, so it's not like Amazon can outcompete merely with a sprinkling of magical internet pixies. Also, they fixed the VAT problem, so Amazon's other main competitive advantage (massive scale tax avoidance) is also no longer available.
I'm sure Amazon could spin up a groceries delivery system which is of comparable effectiveness as the existing major players, but I doubt they'll have any significant advantages.