I don't believe the OP is in fact married yet, but the first impression would be one of giving respect and to be humbled to be invited into another person's home. A firm hand shake to the father and mother and a kind, honest smile will go further than an ill fitting union jack tee shirt. As I remember, the first time I met my wife's mother I held her hand and gave her a kiss on both cheeks (continental style) lol, she wasn't expecting that, that was for sure, but it broke the ice and I never gave her any gifts.