Very similar situation here. My flight just before Christmas was delayed for 12 hours, because of some "anti-icing valve" problem. This may or may not have been the case on the outbound flight to BOS, but my flight was from BOS and delayed simply due to the incoming aircraft being delayed. Aside from that, everything was the exact same wording as reproduced by others here. I will give BA one final chance with a deadline, but it's looking like legal action may need to be taken.
I am a UK citizen resident in the USA (Massachusetts). If it comes to it, can I use the UK small claims process? I find it less intimidating than the US system. Or maybe there's a reason the US system may serve me better?