Welcome to Flyertalk and welcome to the BA forum. Indeed without a date and flight numbers we can't give you any sensible information or advice. However booking on day 355 is usually the most expensive thing to do since only the flexible buckets are added. The lower inflexible buckets typically get added 3 or 4 days later, or some time later in the case of LGW flights. There is zero chance of a service filling up in that first week, and it's just possible the Avios options will open up.