POTC will be open when you are there. The closing of IASW does not impact the crowds for Space Mountain. Different age level.
So here is the situation:
Sunday will be much more crowded than Monday. Sunday is a '43' day (43,000 guests anticipated), and Monday is a '28' day. On Sunday, DL will seem insanely crowded. It won't be, but the crowds in the morning are all bunched in Fantasyland for the two 'premiere rides', Pooh's Hunny Hunt and Monsters Inc. PHH is a MUST, and it deserves its popularity. That is priority #1. Monsters Inc is wildly popular, though I thought it was lame, and I regret having waited on line for it. Most everyone will get a FP for one and ride the other first thing. Your problem will be that, since you are arriving a bit late, you might not get an FP for those rides. Still, I think you will be able to do everything in DL even if you arrive a bit late. That's what I did on a '46' day. Because you live in the UK, getting to Space Mountain might not be so easy for you otherwise. So I say go ahead and do DL the first day.
The advantage here is that the real and true overall priority is Disney Sea, and you will be able to take advantage of a much less crowded day. Get there very early. This park will astonish you, and it deserves a full day.
My best tips:
Indiana Jones and Splash Mountain have single rider lines. Ask for that!
Ask for front row on Journey to the Center of the Earth.
Do not miss Triton's Kingdom, the undergound section of Mernaoid Lagoon. The entrance is easy to miss.
Do not miss Fortress Exploration. Ask for a map in English, it is easy to miss one of the rooms. Hit all the rooms. The map makes for a great souvenir.
There is a lottery for Big Band Beat. Unfortunately, there can be a long line to simply enter the lottery. Enter the lottery as late as possible for the last show. If you go late, the line will have already disappeared, and your chances are the same.
I think they still do a show at Cape Cod Cookout, so that's a good place to have lunch and see a show at the same time.
Don't miss anything. It is all good.