Direct vs connecting flights
 

This is a pretty simple question, but I want to verify before spending a lot of time planning flights. Looking at UA's timetable, it looks like LAX-IAD-LHR on UA 924 gives 5448 miles. If I book this trip using different flight numbers (or 2 separate PNRs), I assume I would get the real mileage (5955 = 2285 LAX-IAD + 3670 IAD-LHR). Is this correct? I haven't checked the fare differences yet - this question is just about mileage.

Your interpretation is correct.

Another solution is to ticket the direct flight as two distinct segments. You will pay about $5 more in taxes--but you will get credit for each segment. This can also be advantageous for upgrades, because one segment can clear before the other. Otherwise the upgrade has to be available for both flights at the same time.

A phone reservation agent can do this for you, or you can use the multiple cities option online to do this.

go for separate tix if not mmuch more $$$
 

since they are running that promo thru IAD, if you ticket separetly, you will get credit for 2 roundtrips from/to IAD. Just something to think about.