In this book set in the far (near?) future, England is surrounded by a wall which is perpetually guarded against ‘the enemy’ who are desperate to get in.
The exact situation is not described, but you get the impression there has been a massive raise in sea levels globally, and that the enemy is desperate climate migrants. Much of the book is a straightforward adventure / romance story, focused on one man who is serving his mandatory two years on the wall. SPOILER His unit allows an incursion, so they are all set adrift in the ocean. Despite it not really going anywhere in terms of plot after this, I still found it a compelling story of an all-too-possible world. Probably my favourite part was how it was well understood by the entire culture that the older generation was entirely at fault and they were ostracized appropriately.