I say NO, and for the following reasons:
1. On the surface, Portishead's music is layed with jazz guitars, samples, turntables, down-beats and haunted house/sci-fi melodies. Anathama's song does not include any of these things and is more organic sounding.
2. Musical backgrounds. Portishead's sound is based pre-dominately in American "black" music. Utley is a jazz guitarist and Barrow's roots are in the hip-hop/DJ scene. In fact, he sites the first Gravediggaz album (and probably the first Wu-Tang) as an important influence in the melodies when he was first writing songs for Portishead. Anathema's musical background is based on more traditionally rock based music, such as The Beatles, Jeff Buckley and (to a lesser extent in recent years) Pink Floyd. These influences shine thru in both of their sounds, and I find them unsimilar.
3. Vocals. This sounds almost nothing like Beth Gibbons. Gibbons has a more up front, tense, confrontational voice. She sounds more, I dunno, bitter or sad. Whoever it singing in this (is it John's sister or Vinny/Danny/Jamie's cousin?) sounds more at ease and comforting. A hint of sadness, but a lot more relaxed than Gibbons. Reminds me a litte bit of Lisa Hannigan, but just a little bit. However I can hear a bit of Janis Joplin in both of their voices. They may have a few similar influences, but sound nothing alike.
I remember all this shit about how Hooverphonic sounded "like Portishead" when ANSSS came out. When I listened to it I was like "No, it fucking doesn't". And for the record, I don't hear any Portishead in Antimatter either. Where the fuck do you people come up with this shit. It's like someone who has no idea of what they're talking about saying that Iron Maiden and Metallica sound the same because they're both hard rock bands. Open your fucking ears!