Forex is basically an OTC market where dealers and brokers negotiate directly and at the same time it should be noted that there is no clearing house or central exchange resent to monitor the activity. The biggest share in trading activity is held by London which as per the last report released contributes to about 37% of the total turnover of the market.
It is because of this dominance only that the quoted price of a particular currency is basically the price of the London market. This is the reason why London is given so much importance when it comes to currency trading.