I think tiaras will be worn, but I was pointing out that an acceptable alternative could be the dress code at the Dutch inauguration, which was also used at the Japanese enthronement, i.e., long dress with orders and hats for the ladies, and military uniform or white tie with orders for men.
Having the British and foreign royals in white tie (or uniform) and MPs in morning dress will look strange, but that is again what happened at the Dutch inauguration. At the Japanese enthronement, I believe all men were in white tie (including the members of the Japanese cabinet), which looked like a better fit.