The answer is so patently obvious I'm surprised it's not been implemented before.......
To save England from invasion by foreigners of all shapes, sizes and religious persuasions.
1. Seal off/brick up the Channel Tunnel
2. Re commission the Martello Towers and man them with the latest in 21st century weaponry
3. Extend Offa's Dyke and float Wales out into the Irish Sea
4. Rebuild and reinforce Hadrian's Wall to keep the Scots out (or in depending on your point of view)
Have I missed anything? Perhaps I ought to have started a new thread jingl but your instructions were too complicated and, besides, this one was just sitting here going nowhere
TGIF!