IMO for the UK having the home fleet based anywhere in the UK would be sufficient to prevent any invasion.....and IMO the Home fleet shuoldn't be able to be based outside the UK, so that would solve the problem anyway [8D]
Riga could not be invaded because of the Islands in the Gulf of Riga restricting access and the passages being easily mined.
However for me the main reason for limiting invasions is simply that almost no-one had the ability to launch meaningful invasions......who's got enough shipping in the Black Sea to invade Sevastopol or Odessa? Turkey? Bulgaria?? They seem unlikely candidates.
The TE has the shipping and the "command of the high seas"..........and in the whole war they launch 4 invasinos - Iraq (Fao peninsular leading to Basra), German East Africa, Gallipoli and Salonika - 1 of these is to an essentially friendly port. Gallipoli we probably know enough about, Fao was essentially unopposed - there was a for there but it seems to have fallen fairly easily. Basra was taken a couple of weeks later after some fairly spirited fighting and "boys own" actions by the plucky Brits and Indians against superior numbers of Turks.
The East African one at Tanga in november 1914 was Lettow-Vorbecks first lesson to the empire! (
http://www.chakoten.dk/tanga_1914.html)
Salonika and Gallipoli are the only actions that come close to GoA scale tho......and IMO they show that there's more to a successful landing than just getting troops onto an enemy shore - both rapidly resulted in major supply problems and required vast nubmers of troops and shipping to keep "viable"
I guess if the supply at any invaded port was 0 (zero) then that might set up the required level of difficulty for the CP - it would ahve to get troops there, then maintain sea supply so they would be effective, plus you should have to have some sea supply to invade - ie you shouldn't be allowed to invade if your troops will be at 0 supply when they get there. I don't know what it is at the moment, but starting at 0 would require considerable naval resource and present a suitably high bar IMO.
Oh and the Russians should get a dreadnought in each of Sevastopol and St Petersburg during the war too......4 Gangut class BB's were commissioned to the Baltic Fleet by December 1914, and October 1915 saw 2
Imperatritsa Mariya class BB's commissioned in the Black Sea Fleet