The price absolutely cannot be guaranteed with an instant order because pricing changes all the time. Even a slow network connection on your end will mean a real-time price (which FT doesn’t currently offer because of the cost to them) will be different to the execution price. Some stocks change prices every few seconds or more and it may take longer than that to execute the order. Thus, the price changes.
Instant orders are not meant to get you the best possible price. They satisfy the “I must have this NOW” feeling. A limit order means you won’t pay more than the price you set. And yes, this means Plus. That’s the reality of it all. Some providers don’t charge for limit orders and if you feel aggrieved/that this is unfair, you have the option to go elsewhere.