Similar situation here - EDI-LHR-FRA-EWR - spending almost 4h in LHR waiting for flight to FRA then 1h connection in FRA. This is despite there being a LHR-FRA flight after a 2h wait, so I'm going to be stuck in LHR seeing a flight depart that I could be on, and then having to potentially rush from mine to make the EWR flight (from what I've heard the LHR arrivals are often late). Doubly-frustrating as I'm flying in F with LH, so don't get to really enjoy the FCT!
Tried ICC, PTS, DC team - none had any luck making the change, apparently the LHR-FRA-EWR bit is a 'married segment'
Good luck at IAH
xcalx!