As an outsider and general LHR user I think BRS deserves credit on how the airport has developed over the years.
What still makes me choose LHR over BRS is the time it takes to reach the airport from around junction 15 of the M4. Especially getting through Bristol area and then up to the airport. As a BA gold card holder I have some perks in choosing LHR but have often thought on using the "orange" on certain routes. It's the travel time that proves to be the down side. Last attempt I was very close to missing my flight to INV and BRS security lines were not very good.