How many hours flight is it from Houston to London?

Not an easy question, because there are so many variables. Assuming a non-stop flight at an average airspeed of 450 knots, the flying time would be approximately 9 hrs 20 min. The actual time will depend on aircraft type, actual speed calculated for maximum fuel efficiency, altitude, wind, routing and other factors. The return will take longer since you are flying against the wind flying east to west, not with the wind, west to east as with your question. If you were lucky enough to be flying in an SR71 Blackbird, you'd get there a bit faster, since its speed is 2,070 mph. Since the distance is 4846 air miles, you'd get there in about 2 1/2 hours plus the time needed to slow down a bit for refueling. (It set a record for NYC-LON of 1 hr 55 min, and London to Los Angeles in 3 hrs 48 min. Average speed 1,435mph.)