Burger King complaint: double whopper 8.99 meal says try this as plant based

I ordered 2 plant based double whopper meals, Normal double whopper meals says price 8.99. a sign says on the picture next to meat version. try this as plant based. I was charged 20.98 for 2 plant based double whopper meals. I said you have charged me wrong. There is nothing to say it is any different price to the meat version. I was informed it is an extra 75p per plant based pattie. so an extra 3 pound for my 2 plant based double whopper meals. I said this can’t be right as there is no where, any notice or price list saying this anywhere in the shop. surely by law the price should be shown and this is unacceptable that it isn’t. especially as it says 8.99 per meal with a little sign saying try as plant based. The man could have said any price or amount extra as there is nothing stated on the menu or on any signs in the shop. I think 20.98 is a disgrace fo 2 x double whopper plant based meals. and I will be telling all my vegetarian friends and family to avoid Burger King in the future, for the excessively over priced meals, especially as they shop at Burger king regularly. Very unhappy customer Dean