Well, the "right" way is to learn to mod using the DB files, edit the blockstate in the London Region and recompile the scenario...
The QDM way [Quick Dirty Method]
First, know that London Region is UID=1246 [use the 'debug' mode from main menu, hover mouse over region to find the UID]
Scenario files are located in WIA\Scens
Make a copy of the .scn file you wish to change
Open the file with any text editor [I use ConText, as it has many nice features]
Search for:
UID=1246
CivLevel=3
Forage=0
Loyalty=0
VP=0
[color="Red"]BlockState=1[/color]
TransportLvl=5
SupplyStock=0
AmmoStock=0
PowerStock=0
SupplyProd=0
AmmoProd=0
PowerProd=0
MoneyProd=0
WSUProd=0
ConsProd=0
CityStyle=0
Change the text in red to [color="red"]Blockstate=0[/color]
Save and go for it....
Note: The European regions are blocked because the AI tends to see them as prime targets, especially the Capitals [London, Paris], so if you play vs. the AI, you may find Athena stalking these regions.....
Good luck.