I agree—LHR and LGW are the only viable option. Of the two, I personally prefer LGW. Easy access to 20 minute express trains into the city, better designed terminal complex and much less crowded. Drawback—not such great connections to other cities.
LHR is a mess. It is short one runway causing long delays, tremendous walks for passengers to get to the tube station that only starts operating to 0530–early checkins have an expensive cab ride. The terminals have been cobbled together over the years so there is no fast way to change terminals and then you have another security check. Passengers tolerate it because of lots of connections to the Middle East, Africa, and Eastern Europe.
If you are looking for just an origination/destination airport. I would vote LGW.