We made a special trip to London to find a flat about 3 months before our move. Glad we did too because the photos on the Internet agencies are very misleading. Like they dont tell you that the great flat is surrounded by council housing (i.e. public housing) or is overtop a 24-hour discoteque. But if you cant afford to fly beforehand, I strongly recommend using an agency. They can guide you to find the property that's right for you. We used Foxtons, probably one of the more expensive agencies but well worth the cost.