The Polynesian is a beautiful hotel and you've made a fantastic choice! The Polynesian offers three types of standard room - garden view, lagoon view and Magic Kingdom view. The price difference is approximately $100/night extra for the lagoon view and $150/night extra for the MK view. The lagoon view rooms do look towards the Magic Kingdom, and will have fireworks views - but the castle is likely to be partially blocked by a tree/foliage. The MK view rooms offer direct/clear views of the castle.
The lagoon and MK view rooms are popular, and there is a good chance you'd be disappointed if you left it until check-in before upgrading. Who have you booked with? A UK tour operator may be able to upgrade you, but check the pricing they offer carefully. If you've booked direct with Disney they'll be able to do the upgrade more easily.
Regarding the in-room flowers & wine, the Disney Florist could do this for you, or you might be able to arrange something once at the resort, via the concierge.
Have fun planning!