Tatler magazine, a U.K. society and style guide, once claimed that Caffè Nero has "The best espresso this side of Milan". Caffè Nero loved the quote so much that they stamped it on the back of all their employee T-shirts. But talk about a back-handed compliment. But talk about a back-handed compliment. In my experience, Milan has some of the weakest and worst espresso - and some of the most inexperienced and oblivious baristas - in all of Italy. (An hour’s drive to nearby Torino, for example, and the espresso is noticeably better.)
Caffè Nero has two Heathrow airport locations at Terminal 1: one small kiosk just inside the terminal, and one large café deep in the terminal’s mini-mall. They claim to charge the same as their in-city locations (1.30£, or 3€ or $3 U.S. - be wary of the clueless tourists in line asking for change in their own currencies). In the U.S., I never find the espresso quality at airport versions of cafés to come close to their normal retail locations. However, that's much less of an issue in Europe. And I was given no reason to believe that the espresso quality at the airport Caffè Nero was significantly different from what I might find at, say, their Chelsea location.
Using a four-group E91 Faema Diplomat (at both locations), they serve espresso as a double shot by default. It has a richer, dark brown crema that's not terribly thick. Bodywise, it's somewhat watery and thin considering. Flavorwise, it is quite bitter and dark in the Martha & Bros roast style: smoky tobacco with some ash and pungency. Even when you order your shot as a ristretto, it comes shorter and has slightly better body but still falls short on the same crema and flavor.
While I would order one of these over anything in, say, a Starbucks anyday, I wouldn’t go out of my way for a cup. Even if flights through Torino are far more impractical.