Defineately travel QM2 but do remember many wheelchair bound people do fly.I work for a major airline and see a great deal of them flying all over the worl.
The Queen Mary two sails directly from the UK to New York on the trans atlantic crossing, you can sail both ways. This may be an option for your mother. Its also a very nice ship and beats flying any day.
The Queen Mary 2 does a 6 night transatlantic cruise nearly all year round.
Having made 25 cruises on several lines I find Celebrity are brilliant for wheelchair passengers. Finding a cruise from/to the UK could be a problem. Next choice would be Cunard.