That's right from LAS, if you're flying in an easterly direction you can't go back to LAX or SFO to get to LHR, you would have to go via ORD/EWR/IAD. That is one of the "problems" with this fare. Well, by that I mean: "things that make this fare way less attractive to purchase for the price - might as well buy a Star1 and do it that way for mostly the same cost (unless you're talking the D class one, which is significantly cheaper than a CRWSTAR1, ex-Australia).
Another option to do this:
enter the US at, eg: YVR, then fly YVR-SEA-PDX-SFO-LAX. Buy a cheapie LAX-LAS, say on Ted as a seperate ticket, then continue your UA/SQ tix LAX-LHR, or even better, go LAX-SAN, then pick up UA930 (I think it is) which tags SAN-SFO-LHR, but reserved as SAN-LHR. You've just gone in a straight line down that coast(which you can do, provided it is same direction of travel) and then get to LHR that way.
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RichardMEL, UA 1P (It's Time to Fly)
A Star Alliance Member.