You won't have been charged £1.85 for the orange juice - it just looks like that on the receipt. They seem to list some of the items at full price, some as free, and some at prices that seem to bear very little relationship to anything else, and it all gets evened out at the end. A happy meal is usually around £2.49, so if the Orange juice was £1.85, you may find the burger is 79p and the chips are £0.00, or similar.
You can't take an item away from the meal or the till won't count it as a meal and you'll actually pay more. You can tell them not to put the toy in the box if you don't want it, but it won't save you any money. The price is a set "saver" type price you get for buying those items together.
My DDs work there (one is a manager) and a lot of people find it hard to grasp. I don't blame them! If you only bought a happy meal and nothing else, however, a mistake has been made if you paid £4.85. A happy meal doesn't cost anywhere near that much!
Sit down means nothing. McDonald's have to know who sat in and who took out for tax purposes but it won't have affected what you paid.