taralluccio
Oh how I, Hand Solo and GS-Alpha wish..
The cheapest buy is usually the ID90 ( certainly for flights ex-UK), which often works out as being comparable to "firm" fares with the LoCos on the same route - I've just bought tickets for my March commute with EasyJet - they are cheaper than doing an ID90 with BA on the same route.
As for upgrades, no, not automatic, even when in uniform, even with seats available up front, you will only get upgraded on an ID90 if you are dealing with "sympathetic" crew. When you head down the back it does lead to some surprised comments from the general public about how BA treat their staff - BA's solution would no doubt be to ban travel in uniform.