To get a grasp on what a margin call is, you should understand the purpose and use of Margin & Leverage. Margin & Leverage are two sides of the same coin. The purpose of either is to help you control a contract larger than your account balance. Simply put, margin is the amount required to hold the trade open. Leverage is the multiple of exposure to account equity.
Therefore, if you have an account with a value of $10,000 but you would like to buy a 100,000 contract for EURUSD, you would be required to put up $800 for margin in an UK account leaving $9,200 in usable margin. Usable Margin should be seen as a safety net and you should protect your usable margin at all costs