Since both flights are on UA, this question might be better answered in the United forum where people are more familiar with connecting on UA at IAD and IAH.

Both IAD and IAH are big airports, so getting from one flight to another can be messy. For the TPA-IAD-LHR flight, although UA allows a 40 minute connection, I definitely wont do it myself. Especially when the 2nd flight is the longer/international flight and possibly the last one for the day. Even if everything goes well, by the time you get to IAD, your LHR flight is already boarding.

When I fly to any destination (XXX) in Europe from SFO on UA, I would much rather take the SFO-FRA on a crappy old 747 and connect FRA-XXX, even with a short connection in FRA. Even if the SFO-FRA flight gets delayed, chances are there will be several other FRA-XXX flights that day they can accommodate me on.

I try to avoid the SFO-ORD_XXX, SFO-IAD-XXX ,and SFO-EWR-XXX, especially with a short connection, because I don't want to get stranded in IAD or EWR incase of weather issues. On the way back I don't mind connecting in IAD, EWR, or ORD, because there are many flights from these cities to SFO. This is just a personal preference.

When flying from ONT, have you thought about taking a direct LHR flight out of LAX? I live in San Jose. For international flights, I'd much rather fly direct out of SFO than fly out of SJC and connect somewhere else.