Here goes, step by step:
1) Open Marla Singer's map (or any other) in the Editor. All the Player Starting Locations appear blue.
2) Right click on any starting location (for example, the one where London would be) and choose Reassign.
3) A window saying Set Tile Owner appears with the default at "None". There are 3 more options below it, they say (faded), Barbarians, Rome and Player 1. Click on Rome and from the drop-down menu choose the correct Civ for that location, in this case England.
4) Once you click OK, the color of the starting location should turn from blue to orange indicating that the location is now reserved for England.
5) Repeat the process for every other Civ.
Have fun