I've never seen either the capsules or the machines at LHR T5 or T3. I haven't been to Terminal 2 in decades so CWS may have better luck there. There's nothing on the Heathrow Boutique (
http://boutique.heathrow.com/) or the Nespresso (
https://www.nespresso.com/uk/en/store-locator#map-intro) websites for Nespresso.