Thanks for the additional replies. From what I understand of the posts, it appears that BAEC Silver will be allowed access to BA Lounges even though not flying on BA for the day - any oneworld carrier will do; pending lounge operating hours and accessibility of the location.
Regarding hours, do they stop admitting people at a certain time before the last BA flight departs (i.e. last BA flight 10:30PM, no admittance after 10:15)?