We used to go Dublin - LA - LV so that you could clear immigration in Dublin. Relatively OK flights with aerlingus. The last few times we've gone via Gatwick with Virgin as (in my opinion) clearing immigration in Vegas is much less stressy than in most other US airports. If you go Heathrow - LV with BA, T5 is a much nicer place also to hang around in than most other airports. In my opinion.