You'd be surprised what other options are available to you from the UK. Just ring one of the cruise agents on there 0800 number and ask what is available for your dates.
Have you thought of going on a fear of flying course?.THey have good results and i know several people who have done i.
You could go on the transatlantic crossing and get a ship from NY back through Newfoundland to Southampton.
I am not sure but do P&O do a round trip from Southampton outside of the med.
You're right - cruises from/to the UK around Jul/Aug are predominently Med/Canaries/Baltic/Fjords etc... If you really don't fancy any of these options why not a transatlantic to New York aboard QM2 (Cunard)? You'd have the choice to extend your stay in New York, or perhaps travel around a bit to Philadelphia, Washington DC or Boston? Packages can be tailor made to suit.