I don't know when you're looking, but with a 3 wk advance purchase and a one-week stay (Mar 6 - March 17), a round trip LAX
is 445 on virtually all carriers, London is in the 600 range.
With a one-week advance purchase and a one-week stay (Feb 20-27), United is 900+ to LHR
, and the lowest nonstop (Virgin) is 590. Delta will take you r/t to Hawaii for 447 on the same dates.
If you're leaving tomorrow for a week, United will take you round trip for 532 nonstop to HNL
, whereas the cheapest nonstop to LHR
is on Virgin for 597 (the rest of the carriers except CO
being over 2000).
All these are nonstop fares, from Orbitz. Maybe it's just the carrier you're choosing and the specific dates that causes what you see as the irregularity.