The spread for EUR/GBP rises to 0.8532-0.8533 and you decide to sell your euros back into pounds at the bid price of 0.8532. The €10,000 you previously bought is now therefore sold for £8532. Your profit on this transaction is £8532 minus the original cost of buying the euros (£8415) which is £117. Note that your profit is always determined in the second currency of the forex pair.
The need to exchange currencies is the primary reason why the forex market is the largest, most liquid financial market in the world. It dwarfs other markets in size, even the stock market, with an average traded value of around U.S. $2,000 billion per day. (The total volume changes all the time, but as of August 2012, the Bank for International Settlements (BIS) reported that the forex market traded in excess of U.S. $4.9 trillion per day.)