I can't see how you could possibly know that they would become unavailable after you take the battle tower.
You are given a warning before entering the summoning chamber.
To be fair to him, the warning was about Broken Valley not Sentinel Islands as far as I can recall. But one can only assume the inevitable fate of both game Zones once he/she realizes that Divinity II is not an open ended/sand-boxed game and follows a rather arcade pattern. Once you reach a new high, new areas/scenarios become available rendering the old ones unreachable. Farglow is lost once you get to Broken Valley, Broken Valley and Sentinel Isles are out of reach (mostly) after receiving Battle Tower. Only Orobus Fjords remain accessible (at least to a certain point) throughout the game and even then it will become unavailable once you enter Hall of echos. It's a game where it's pretty easy to lose on lots of things which is definitely a annoying experience but on the flip side you don't need everything to complete the game and get lots of fun.
