You will lose any TPs you have on 8th September. You will not lose any miles. You will then have a new membership year until 8th September 2008.

Card Expiry is irrelevant (and useless) if you're a Blue (since they don't send you new cards now) and only of interest to Silvers/Golds if BA fail to send you a new card on time (which is why its expiry is a month after your membership year ends - though mine appears to end at the end of the month after my year ends, not just exactly one month after - e.g. if year ends 8th September, card ends 31st October...a fair bit of time of extra status if, heaven forfend, one doesn't make the grade one year!).