Can I recommend St Peter's Without. There are a couple of variants, a regular, dark bitter and a lighter cross over bitter called Gold. I think there is also an organic version.
Comes from the fantastic St Peter's Brewery in Suffolk. The owner was diagnosed with cancer and told he couldn't drink alcohol, so he set about making an AF beer that genuinely tasted like beer. He has very nearly done it. As others have said, it's hard to do sessions on AF beer for some reason, but they are so low in calories, it wouldn't really matter if you did.
St Peters is a great brewery. One of the few good home brew kits as well.