It does seem hard to imagine that there could be a non-stop flight to the US from Bristol, one that went via Glasgow may be more viable.
The runway length and airfield elevation make it operationally more difficult for such a flight.
The AA flight out of BHX , which has a considerably longer runway than Bristol had problems with weight if the temperature rose above about 22 degrees in the summer and had to off load freight or baggage to get airborne.