Check in desks at IST are at Zone G, located towards the far end of the check-in hall (turn left after the entrance security check) and open at 08:00. As a CE passenger you can collect a fast track security sticker and lounge pass at check in.
Boarding is usually at Gate 504, which is a coach gate. Boarding starts at around 09:00. Group 1 passengers are loaded onto a dedicated bus and driven to the awaiting aircraft ahead of the other passengers.
Be aware, because the BA675 night stops at IST, the CE meal service is not the normal three course meal. Instead, you should expect to receive a Band 1 hot breakfast - choice of full English or cheese omelette with a small yoghurt pot and pickings from the bread basket.
Enjoy Istanbul - it is a truly amazing city