Use a main post office, first class this is the most likely chance of getting post to destination in time. Avoid posting in Pillar Boxes these sacks, once collected, will not be done as fast as the ones collected from main post offices. Reason - the ones at main post offices are more likely to have registered and special deliveries i.e. priorities.
Mail is being sorted this weekend but managers are doing the sorting. Posties will be back in full swing on Wednesday - so post on Wednesday at a Main Sorting office.