Um, black flowery high wasted skirt and leggings?

Since your boyfriend's parents are into style, I think you should go with the floral high waisted skirt, leggings or stockings/tights, singlet top, and cardigan. I think that outfit might make a better, or more fashionable, impression than jeans, a tee, and trainers. But, the main thing is that you dress comfortably and be yourself. :)