Train from Edinburg to London- how to buy forward facing seats?

When booking online, it allows you to select seats on a seat map. The seat maps shows arrows on both sides of the carriage marked "direction of travel". The seats are shown, with the rounded edge being the front (ie the square edge is the back). Coming out of Edinburgh left side is best as it faces east (best view for Durham Castle and Cathedral, and also costal views just before Berwick-upon-Tweed).