Firstly let me say that your furkids are ADORABLE! I love their markings! The little one is just precious! Good for you wanting to protect them!
My boys are indoors only. I first got them when I lived in SF so they are city kittys! I agree 2000% with Cindy. Indoor is my first choice.
I do know that it's not common to have an indoor cat especially in the countryside in England.
I know I've seen ALOT of cool enclosures here on PT. Some homemade, some store bought. I'm sure you'll get alot of good advice/suggestions here.
Kisses and scritches to you cute kids!
Claudia