The rules of any of the expansion sets replace the rules of the game within the scope of those expansion rules. You’re correct in assuming the British get a special price for radar when using the Sealion Expansion. You will find these types of special costs in many other expansion sets as well. Unless you’re creating your own house rules, use the rules of each particular expansion in your game.
Requirements for facilities
Hey I was looking for the general requirements of placing facilities and all I found was that factories can be placed in land zones with IPP value and that’s about it. Does that mean that even with a surround city you can just rush a fort (given you have the improved construction tech) in that surround city?
The encirclement of the city affects the defense value of units, but I don’t think there is a rule that prevents the surrounded city from building new factories, new units (if it has a factory) or facilities, including a fortification.
Another option could be to add an airbase to make it not surrounded…
You can place a fortification in any territory that you own. Note that when building a minor factory outside of your Home Country, it has to be in a supply path to a major factory in the Home Country and not just a territory with an IPP value.