Please help with Outfit for friends wedding...I'll be 36weeks pregnant! UK ONLY?

I was going to say a maxi dress but if they don't suit you what about an empire waisted dress, maybe in a floaty fabric like chiffon. The high waist will accomodate your bump and flatter your lower half. They look good with heels, and a dress will be much nicer for a wedding than leggings and top (and will give you an excuse to wear something else)