As a churchman with a fair amout of experience here, I would advise that this is semi formal, the equivalent of a suit and tie, definitely not tuxedo. That said, different congregations have a different worship culture when it comes to attire, but wearing either Argyll, black or tweed, with a tie will suit no matter what the climate. Dress sporran if you like plus flashes and you should be a welcome addition to the entire worship experience.