Based on previous experience at Waterloo the busiest periods for check-in are Friday afternoons, Saturday mornings and Sunday evenings. I would recommend turning up at least 45 mins before departure if you haven't already collected your tickets. There will be plenty of machines for you at St Pancras to collect them from.

Regarding passport control at St Pancras - after passing through security this will involve two inspections. Firstly your passport will be checked by British Immigration officers. You will then proceed to another desk where your passport will be checked again, this time by French Police officers. It remains to be seen if passport control by British Immigration officers will be on a permanant or a part-time basis.