You should do a search on this on the BA Forum as I know this has been discussed. As I recall, there are not any great discounts in doing it this way. And I have to say, WT+ and NCW run very full on the flights I have been on out of LAX, so I would not count on anything being available. In my opinion, it is well worth the money to at least upgrade to WT+ by reserving in advance. It is not a huge amount and certainly makes the trip much more comfortable.