You can enter the custom battle scenes by using the quick scene chooser. Click on 'Play Tutorial' and 'Scene Chooser'. Another way is to copy&paste the terrain code of the battlefield you want to enter into the scene entry in the scene.txt and rename the battlefield like the scene you want to use.
The advantage of the latter method is that you get to choose at which time of the day you enter the scene. The scene chooser makes you enter the scenes only at sunset.
Here is an example:
scn_1257_combat_mountain_1 can be found in the SceneObj folder, like all other scenes, and you can rename it to scn_london_center. Before that rename scn_london_center to any other name like scn_london_center_original. Both scenes already share the same terrain code '0x00000000300005000009de7900000f300000446800005b1f'.
Otherwise you need to change the terrain code in the scenes.txt. If you use a scene with a different terrain code, you need to overwrite the terrain code of the original scene (in this case London) with the new terrain code (in this case combat mountain 1).
You can then enter the scene normally.
I can't tell you, which scenes you like the best, just enable the cheat mode before starting the game and hold 'ctrl' and left click on any position on the strategy map to go there.