Based on my own personal experience, I've had no problems with being upgraded flying LGW-IAH or vice versa when booking a 'K' class economy ticket. They've usually happened for me at the 26-30 day window.

Boo Boo Too have you considered flying LGW-CLE (personally, I wouldn't dream of flying LGW-CLE or vice versa unless I was guranteed a BF seat at the time of booking as I wouldn't relish 9hrs in Y on a isngle aisl aircraft) or looking at a CO ticket on a VS codeshare out of LHR? From memory, the timings of VS LHR-EWR flights are slightly different to CO's LGW-EWR flights, so you may have less of a layover.

HTH

Steve