I actually happen to agree with you, and Raymond Madsen for that matter. I think if Mary Blackwell had married a different John Brockett and stayed in England, then there would be some kind of record there. However, it would be nice to find indisputable evidence to prove it.
Unfortunately I've visited the Wallingford CT town clerk's office and there is no mention of John's wife in any records there. There simply is no reference to her name anywhere in local or state records.
Still, the Brockett.info site is an excellent family history no matter which branch John Brockett actually came from. We may never know for sure. Until someone proves otherwise I'm sticking with John Brockett of Caswell.