Are these ER1149 certs you're using? If so be sure that they are issued in "J" class when you trade them for tickets. And yes, these type of certs will go to the bottom of the list in priority. They are categorized as "S8" which falls behind all revenue passengers (naturally) and also behind employees (S4 and S5) and other airline employees and "buddy passes" (S7).
LAS is bad for non-revving any time, unfortunately. And US Airways' limited presence in/out of Vegas makes it difficult if you're trying for an onward connection. I know some people that leave the west coast on a red-eye the previous night in order to make European connections. For peace of mind you might want to keep an eye on fare sales and buy something confirmed (personnally I've only had one successful non-rev into Vegas...for the return I had to buy a ticket on WN to PHX where I caught the US flight to PHL. Other times I've non-revved to LAX and bought tickets on HP).
LGW on the A330 is a nice experience, even in economy. Not sure what the loads look like however Saturdays are typically heavy with business travelers that want to have Sunday in Europe to recuperate before going to work.
Everything considered, though, mid-September is probably not going to be as busy this year since many people aren't flying. And, ultimately, anything can happen when non-revving! So watch the loads - you can call US Airways reservations for the number of open seats. Just make sure you "meal list" for the flight and identify yourself as a non-rev on an ER1149 to the agent.