Once an order is on the books, it can only execute at the rate at which it was placed. So once an offer to buy 1 coin at £1000 is placed, whoever takes it -- however they take it -- is going to pay £1000 if they buy 1 coin.

While an offer is being placed, it can cross with offers in the other direction if the rate is equal to, or better for the placer than, the rate of the offer being placed. Better offers are always taken first to reward those who placed the best offers.

If the offer doesn't fully cross existing offers, the remainder is placed on the books at the chosen rate. The only difference between a buy and a sell is the amount of the offer placed at the end.

For example, if I place an offer to buy 2 coins for £1000 and am able to buy 1 coin for £400 when crossing existing offers, the remainder of my offer would be placed as 1 coin for £500. If that is fully taken, I'll wind up buying my 2 coins for £900, that is, buying what I asked to buy, but paying less.

If I place an offer to sell £1000 for 2 coins and am able to sell £400 for 1 coin when crossing existing offers, the remainder would be placed as £600 for 1.2 coins. If that is fully taken, I'll wind up selling my £1000, but getting 2.2 coins for it, that is, I sell what I asked to, but get more.