I've never had the opportunity to take the Concorde, but I do have a couple of functional observations. Since flying supersonic over land is a no-no, why are you taking Concorde?

Here are some statistics (note that I don't take into account head/tail wind, takeoff/landing velocities, other items--this is not scientific at all):

1. The average cruising speed of Concorde is 1350 mph.
2. The average cruising speed of a B767 is 530 mph.
3. The average cruising speed of a B747 is 570 mph.
4. The average cruising speed of a B777 is 555 mph.

So, the following journey times would be observed:

1. LHR/JFK (BA/Concorde) 2.5 hours.
2. JFK/LAX (UA/B767) 4.6 hours.
3. LHR/LAX (UA/B777) 9.8 hours.
4. LHR/LAX (BA/B747) 9.5 hours.

If you had zero ground time in JFK, this would be a journey time of 7.1 hours. This is a couple hours shorter than either the BA or UA option non-stop. However, we have the following problems:

1. There is ground time at JFK (including clearing customs and immigration).
2. We add 45 minutes _if_ you happen to make the last connection at JFK.
3. We add nearly 2 hours if UA kept the 7:30 flight.
4. We add 13 hours if you miss the last connection at JFK.
5. Single ticket LHR/JFK/LAX with BA/UA would not be a legal connection with a mere 45 minute gap. You could risk it if the tickets were split into two.

You're lucky that BA and UA share the same terminal. If you didn't I'd say you'd have no chance of making any connection in that amount of time (unless Concorde was really early).

Of course, I'd love to fly Concorde at least once in my lifetime. But, I figure I'd fly when it actually saved me a lot of time rather than just as part of a longer itinerary to the West coast. I loved BA's adverts about arriving before you left.